Most of this guide is taken from Elitist Jerks but, figured it'd be easier to find here.

The Basic Stats

Strength (str)
1 strength = 2 attack power (AP) = 0.14 dps weapon (white) damage.
2 strength = 1 block value before talents, 1.3 block value with the Redoubt talent
The Divine Strength talent increases total strength by 15%.
Agility (agi)
52.08 agility = +1% dodge chance. This is somewhat less efficient than Dodge Rating at increasing your dodge chance (39.35 dodge rating = +1% dodge chance), but with Blessing of Kings the number decreases to 47.35 agility, which makes it about 80% as efficient as agility.
52.08 agility = +1% melee crit chance.
1 agility = 2 armor.
Agility is often overlooked as a tanking stat, but it's actually an efficient way to get avoidance, mitigation, and threat from one stat.
Stamina (sta)
1 stamina = +10 total health (hp).
10 stamina = +3 spell power (SP) from the Touched by the Light talent.
The Sacred Duty and Combat Expertise talents increase total stamina by 8% and 6% respectively, for a total increase of 14.48% with both talents. If you're buffed with Blessing of Kings as well, then each additional point of stamina increases your total health by approximately 12.6.
Intellect (int)
1 intellect = +15 total mana. This increases the size of your starting mana pool and the maximum amount of mana you can store at any point. It also increases the rate of mana regeneration from effects that restore a fraction of your mana pool, such as Replenishment, Blessing of Sanctuary, and Divine Plea. This is a nice effect, but it doesn't make intellect worthwhile as a stat for tanking gear.
166.67 Intellect = +1% chance to crit with spells, but all Prot paladin offensive abilities except Exorcism use the melee crit rate.
Spirit (spi)
Increases mana regeneration outside of the five-second rule. As a tank, this effect is negligible (and frankly it's negligible for all paladin specs).

Combat Ratings

Defense Rating
4.92 defense rating = 1 defense skill (If you're new to these stats, pay careful attention to the difference between defense rating and defense skill.)
25 defense skill gives:
+ -1% chance to take a critical hit from melee or ranged (non-spell) attacks.
+ +1% chance to be missed
+ +1% chance to dodge, parry, and block (each).
Mobs have a 5% chance to crit a player of the same level with a fully trained defense skill (400 defense skill for a level 80 player). Each level of difference increases the mob's chance to crit by 0.2%. Hence:
+ A level 83 mob (e.g., a raid boss) will have a 5.6% chance to crit a level 80 player with 400 defense. Accordingly, the player will need an additional 5.6 * 25 = 140 defense skill, or a total of 540 defense skill to be uncrittable by raid bosses. This is equivalent to 689 defense rating.
+ A level 82 mob (the highest level found in heroic 5-man dungeons) will have a 5.4% chance to crit a level 80 player with 400 defense. Accordingly, the player will need an additional 5.4 * 25 = 135 defense skill, or a total of 535 defense skill to be uncrittable in heroic 5-mans. This is equivalent to 664 defense rating.
Players with enough defense to be uncrittable don't benefit from the crit-reduction aspect of additional defense skill, but they still gain miss, parry, dodge, and block chance as defense is added.
+ 25 defense skill gives +1% to miss, parry, dodge, and block. This requires 123 defense rating.
+ Hence, adding 1% total avoidance (blocked attacks included) requires approximately 31 defense rating.
+ Adding 1% full avoidance (blocked attacks excluded) requires 41 defense rating.
Increases to the chances to parry, dodge, and be missed generated by adding defense count towards the diminishing returns on those stats (see below). However, there are no diminishing returns on the chance to block.
Dodge Rating
39.35 dodge rating = +1% chance to dodge.
Dodge rating is affected by diminishing returns in WotLK: the more dodge you have, the more dodge rating is required to add each additional percent chance to dodge. This is a rather complicated system, but effectively it means that dodge rating is probably inferior to defense rating as a means for adding pure avoidance.
Dodging opens an opportunity for Overpower mechanics to hit you for a large amount of damage. This is primarily a concern in PvP (against warriors) but there are a very few raid bosses and mobs that have an Overpower-type mechanic. This is not generally worth worrying about, but it may come to bear on specific encounters.
Parry Rating
49.18 parry rating = +1% chance to parry.
Like dodge rating, parry rating is subject to diminishing returns, but it's already inferior to defense and dodge for adding avoidance.
When an attack is parried, your next attack will happen more quickly. (This is why parry is more "expensive" than dodge.) I won't go into the exact mechanics here, except to note that (a) this effect is more pronounced with slower melee weapons, and (b) weapon-based threat is generally not significant enough for this to make much of a difference, and hence it's not worth considering when gearing for tanking.
Block Rating
16.29 block rating = +1% chance to block.
This does not suffer from diminishing returns.
Block is far "cheaper" than dodge or parry or even defense per point of avoidance; however, blocking only absorbs an amount of damage equal to your block value whereas m/p/d avoid all damage. (It does, however, provide threat when Holy Shield is active.)
Hit Rating
32.79 hit rating = +1% chance to hit with melee or ranged attacks (-1% chance to miss).
+ For our purposes, this applies to melee swings (white damage), Avenger's Shield, Hammer of the Righteous, Shield of Righteousness, Hammer of Wrath, and Judgements.
+ Against a raid boss, melee and ranged attacks have a base 8% chance to miss (this is changed from TBC, where ti was 9%), so 263 hit rating is required to eliminate all melee misses if there are no other bonuses to hit.
+ Draenei have a racial aura that gives +1% to hit. This reduces the requirement for melee hit-capping to 230 hit rating if you're a draenei or have one in your group. (This is group-only buff, not a raidwide buff.)
26.23 hit rating = +1% chance to hit with spells. For our purposes, this only applies to Righteous Defense.
+ Against a raid boss, spells have a base 17% chance to miss. In the past, some bosses that required frequent taunting and tank-switching (e.g., Nalorakk, Mother Shahraz, and Brutallus) had only a base 9% chance to resist a taunt. It's not known whether that will continue to be the case.
+ The Draenei racial aura also applies to spells, as do the Misery and Improved Faerie Fire debuffs (+3% to hit with spells, only one of these can be present).
+ The Glyph of Righteous Defense reduces the miss chance of Righteous Defense by 8%.
+ It's not worth going through all the combinations of buffs and debuffs and the hit rating required to cap spells for each, but a few are worth noting:
# If you have no other buffs and no RD glyph, 446 hit rating is required to reach the hit cap for RD.
# If you have the RD glyph, only 236 hit rating is required to cap RD. This is less than the hit rating required to hit-cap melee attacks, so once you reach the melee hit cap, the glyph will safely put you over the top for RD as well.
# If you really want to cap RD without using a glyph slot, the best possible case is to have a Draenei around (+1%), and to have Improved Faerie Fire or Misery up on the mob (+3% hit bonus, only one can apply). This reduces the miss chance to 13%, which would only require 341 hit rating to cap (111 more than the melee hit cap.)
Expertise Rating
8.10 expertise rating = +1 expertise (As with defense, this can be confusing, so be careful.)
Each point of expertise reduces the chance for your attacks to be parried or dodged by 0.25% each. Hence, each point of expertise reduces the total chance for your attacks to be avoided by 0.50% (until the target's dodge chance reaches zero.)
32.79 expertise rating = -1% dodge and -1% parry for your attacks.
Human racial bonuses give +3 expertise when using maces or swords (-0.75% dodge and -0.75% parry). This bonus is worth approximately 24 points of expertise rating.
The Dwarf racial bonus gives +5 expertise when using maces (-1.25% dodge and -1.25% parry). This bonus is worth approximately 41 points of expertise rating.
The Combat Expertise talent gives +6 expertise. This is worth 49 points of expertise rating.
Expertise will affect normal (white) melee attacks as well as Hammer of the Righteous (a parry is reported as "Deflect").
Expertise will not affect Shield of Righteousness, Righteous Defense, Avenger's Shield, or Judgements, because these cannot be parried or deflected.
Since parries by bosses hasten their next attack (using the same mechanic as for players who parry), expertise can be useful for reducing this effect and preventing damage spikes.
Prinsea: "The estimated dodge cap is at 6.5%, which would require 26 expertise or 214 expertise rating to achieve. Removing all parries would almost certainly require much more than this, as boss parry rates ranged from 11% - 15%, but the dodge cap could serve as a decent point of reference until we get more WWSes and such for confirmation of the parry cap."
Crit Rating
45.91 crit rating = 1% chance for a critical strike with all attacks and spells.
All critical strikes for Prot paladin offensive abilities do 200% of normal damage.
Critical heals do 150% of normal healing, or 195% with the Touched by the Light talent.
Crits are fun (especially with Shield of Righteousness) but crit rating is generally not an efficient tanking stat. Hit rating is much better, as it increases your threat more efficiently and makes your threat output more reliable as well.
Haste Rating
32.79 haste rating = 1% haste. This increases autoattack speed by 1%, reduces the cast time of spells by 1%, and reduces the global cooldown triggered by all spells (including instant-cast spells) by 1%. This does not affect the global cooldown triggered by special melee attacks.
While it's always nice to be swinging faster, this is not very useful for a prot paladin, since for the most part they're limited by cooldowns on key abilities rather than by the global cooldown or melee swing speed.
It is possible that a large amount of haste might reduce the global cooldown enough to allow a greater variety of ability rotations, and potentially more threat. However, this seems unlikely to be worthwhile, since prot paladins generally produce plenty of threat already, and there are other stats that can more efficiently improve threat output.
Resilience Rating
81.97 resilience rating = -1% chance of taking a critical hit with any kind of attack, and -2% damage done by critical hits against you.
Resilience is almost exclusively a PvP stat. While resilience is more efficient than defense for eliminating critical hits (123 points of defense vs 82 points of resilience to get -1% crit), defense also provides a large amount of avoidance while resilience doesn't. The reduction in spell crit provided by resilience is useless in PvE because mobs can't crit with spells, and the reduction in critical damage is meaningless since a tank will (hopefully!) be immune to crits from mobs in the first place.
Nonetheless, it may be useful to use resilience for tanking on a situational basis:
+ When first gearing up for serious tanking, resilience gear may be useful as a temporary stop-gap measure to eliminate crits while you collect gear with enough defense rating.
+ Fights that require magic resistance may make it more difficult to reach crit-immunity through defense alone, and a few pieces of PvP gear may be helpful in keeping crit-immunity.
+ In cases where you need to tank a not-too-dangerous mob early in a fight and then switch to DPS or healing after that mob dies, PvP gear with high stamina and resilience may be useful. However, this is a role generally better-suited to a Ret or Holy paladin than a Prot paladin.

Other Stats

Armor
Armor reduces all incoming physical damage. While it does not work on magic attacks, it is guaranteed to mitigate all incoming physical damage. It does not rely on "chance" effects like blocking or avoidance, and it works even when you're stunned or otherwise incapacitated. Armor is your most reliable damage mitigation. The amount of incoming physical damage mitigated by your armor can be seen by mousing over the armor stat on your character sheet; this number is usually referred to as the damage reduction, or DR, and is expressed as a percentage.
There is some confusion about "diminishing returns" on armor. The DR given by armor follows a diminishing-return curve, so the higher your DR is, the more armor is required to increase it by 1%. However, the value of each extra point of DR increases as your DR increases. For example, consider an attack that does 10,000 physical damage before armor is considered:
+ If your armor DR is 50% and you increase it to 51%, the damage done by the attack is reduced from 5,000 to 4,900, a 2% reduction.
+ If your armor DR is 60% and you increase it to 61%, the damage done by the attack is reduced from 4,000 to 3,900, a 2.5% reduction.
So, even though your DR% will increase more slowly as you add more and more armor, each extra point of armor is providing roughly the same relative benefit. Hence, increasing your armor is always worthwhile. (For a more detailed explanation with math and such, see Quigon's Protection Warrior Guide.)
The important thing to remember is that while your character sheet shows the changes in the absolute value of your DR, your healers will see the relative change in your DR. For example, if you go from 60.0% DR to 64.0% DR, your character sheet only shows a 4% increase, but your healers will notice you taking 10% less damage. (Actually they'll see even more than that when blocks are factored in.)
Shields have a disproportionately large amount of armor compared to other armor pieces. Hence, almost any shield from a higher tier of loot than the one you currently have will probably be an upgrade, even if the other stats aren't quite what you'd like. Even a shield with caster stats on it may be a mitigation upgrade compared to a lower-tier tanking shield. (Obviously however, you should respect the resto/elemental shamans and holy paladins in your raid regarding caster shields.)
There is a cap on armor DR at 75%. However, this value is rarely seen in normal practice, and can only be achieved through the use of multiple stacked buffs (Improved Lay on Hands, Inspiration, armor potions, etc). In general, unbuffed armor DR values for plate-wearing tanks in endgame gear in TBC were between 60% and 65%.
The following table shows the returns for adding additional armor at a few different DR levels. Note that the additional reduction in damage taken is always relative to the current incoming damage (before blocking).
Armor DR% DR% w/ +100 armor DR% w/ +1% armor
16594 50.00% 50.15% (0.30% reduction) 50.25% (0.50% reduction)
24891 60.00% 60.10% (0.25% reduction) 60.24% (0.60% reduction)
38719 70.00% 70.05% (0.18% reduction) 70.21% (0.70% reduction)
Block Value
Each point of block value causes your blocks to absorb an extra point of damage, and causes your Shield of Righteousness to deal an extra point of damage.
Block value is increased 30% by the Redoubt talent.
Note that strength also increases block value at a rate of 2 str = 1 blkval. If you're interested solely in increasing block value, items with direct block value bonuses are more efficient than items with strength. However, if you're interested in damage and/or threat generation, strength is more efficient overall since strength increases the damage done by other abilities as well (via AP). Overall, pieces with both strength and block value usually give you the most bang for your buck.
Blocking is the last mitigation effect applied to incoming damage. Since armor is applied before block value, increases in armor also increase the fraction of total damage you block as well.

The Hit Table

It's often assumed by casual observers that the different kinds of avoidance are checked in sequence, e.g. first the server checks to see if the mob misses you; if it doesn't miss then the server checks to see if you parry; if you don't parry then the server checks for a dodge, etc. This makes intuitive sense, but it's not the way things actually work.

What actually happens is the server makes a single "roll of the dice" to determine what happens on an attack, and all your avoidance chances, as well as your chance to be crit, are applied at the same time to that one roll. So for example, if a tank is naked and using a trash can lid as a shield, and has a 5% chance to be missed, 5% chance to dodge, 5% chance to parry and 5% chance to block, the server constructs a hit table that looks like this:

01 - 05: miss (5%)
06 - 10: parry (5%)
11 - 15: dodge (5%)
16 - 20: block (5%)
21 - 95: hit (75%)
96 -100: crit (5%)

... and then a single random number between 1 and 100 determines the outcome.

If a tank has a 10% chance to be missed, a 10% chance to dodge, 10% chance to parry, 10% chance to block, and has enough defense to be immune to critical hits, the table looks like this:

01 - 10: miss (10%)
11 - 20: parry (10%)
21 - 30: dodge (10%)
31 - 40: block (10%)
41 -100: hit (60%)

If the tank has Holy Shield active, the chance of a block goes up to 40% and the chance of a regular hit goes down to 30. If the tank has Holy Shield active and Redoubt procs, the chance of a block is 70%, and it is impossible for an attack to hit without being blocked.

The important thing to be aware of here is that the more of each kind of avoidance you have, the more valuable the rest of your avoidance becomes. Every increase in your chance to parry, dodge, or block comes directly out of your chance to take a hit.

Threat and Damage Generation

The primary means for generating threat as a paladin tank is the Righteous Fury buff together with Holy damage. All special attacks for a prot paladin deal holy damage, so effective threat generation comes down to using the appropriate Holy-damage abilities for the situation.

Note that in addition to the 90% threat bonus to holy damage, Righteous Fury also provides a hidden buff that increases all threat you generate by 43%. (This is the same as reducing everyone else's threat by 30%, which is in fact exactly what it's intended to do, replacing the old Blessing of Salvation.) This works multiplicatively with the holy damage threat boost, so in fact Holy damage generates 172% more threat with Righteous Fury than without. Needless to say, this is a huge difference, and if you try to tank without Righteous Fury up, you'll notice the difference pretty quickly.

Shield of Righteousness (ShR): A very large amount of single-target damage and threat for an extremely cheap mana cost. In single-target tanking situations, ShR will produce by far the largest portion of your threat, and should always be part of your rotation. In multi-target situations, ShR can be used to boost threat on individual mob that you may not have locked down. For example, if you pull a pack of 4 mobs with Avenger's Shield, the one mob that doesn't get hit will reach you first (due to the snaring effect) and you can compensate for missing it with AS by whacking it with ShR.

Because it generates a large amount of threat in a single attack, ShR is also excellent for "burst" threat to quickly pick up individual lose adds and other annoying mobs. ShR can be invaluable for situations where you need to pick up one and only one target (for example, when you need to leave other mobs untouched for other tanks to pick up.)

ShR scales with block value, so adding block value is a good way to boost your threat and your mitigation at the same time.

The [Glyph of Shield of Righteousness] reduces the mana cost of ShR to next to nothing. Since mana is rarely an issue when tanking, this is generally not considered very useful, and there are other glyphs that will give you more value from your major glyph slots.

Hammer of the Righteous (HotR): Excellent threat against up to 3 mobs at a low mana cost, or 4 with the major glyph for this ability. Damage per target is not as high as ShR, but total damage against 3 targets is greater than ShR damage.

In addition to generating significant threat itself, HotR also counts as a weapon attack against each target for your currently active seal. This is most effective with Seal of Vengeance, since it allows you to build and maintain full 5-stacks against 3 or more mobs at once without switching your primary target, but is also handy if you're using Seal of Wisdom or Seal of Light to regenerate mana/health. Situationally it can even be useful with Seal of Justice to interfere with caster-type mobs.

HotR will not bounce to crowd-controlled mobs, so you can use it with impunity around CC, as long as you don't throw it directly at a CC'd target.

Since HotR's damage is dependent only on your weapon dps, it scales with attack power, and consequently strength. Weapon speed is irrelevant for HotR's direct damage. The only cases where weapon speed has any bearing on HotR's effect are when it's used with Seal of Righteousness (which deals damage per swing proportional to weapon speed) or with Seal of Light, Wisdom, or Justice (which have a chance to proc proportional to weapon speed). By and large these effects are not considered worth worrying about, and generally you'll get the highest threat from the weapon with the highest dps.

Avenger's Shield (AS): Excellent threat against up to 3 mobs at a moderate mana cost. In general it makes a great tool for pulling packs of mobs, but it can make things difficult when you only want 1-2 mobs out of a larger group.

The 10-second snare effect is either a blessing or a curse, depending on your point of view. In the best possible case, it gives your group time to get CC taken care of, with the bonus that when the CC wears off the mobs are aggro'd on you, and not on the mage/warlock/whatever. In the worst possible case, it means you stand around for 10 seconds waiting for the mobs to get to you, during which time your ranged dps may be building threat but you probably aren't.

The Shields of the Templar talent adds a 3-second silence effect to AS. While this is most useful in PvP, it can also be handy for pulling caster mobs in PvE. Of course, the mobs will also be snared during the silence, so you probably won't be able to pull them very far. You can also use this as a spell-interrupt in a pinch.

In situations where you don't want the snare or silence effects and/or you only want one target, use Exorcism instead.

Like HotR, AS does not bounce to CC'd mobs.

The Glyph of Avenger's Shield doubles its damage but removes the bounce effect, limiting it to one target. This has situational usefulness, so by all means take it if you like it and skip it if you don't.

AS scales with both attack power and spell power.

Exorcism: Very good threat against one mob at range, at a moderate mana cost. Exorcism is a great tool for getting the attention of a single mob, either when Avenger's Shield is on cooldown, or when the secondary effects of AS are undesirable. Exorcism always crits against demons and undead.

The Sanctity of Battle Talent increases damage from Exorcism by up to 15 percent.

Holy Shield (HS): Reactive threat and mitigation. The threat from Holy Shield is not as important as it once was for tanking, but it's still useful as a threat builder, and it remains an important mitigation tool. Even in pre-raid gear, block values can easily exceed 1000, which represents a significant chunk of mitigation. Moreover, blocks generated by Holy Shield will trigger mana returns from Blessing of Sanctuary; it only takes 1-2 blocks for HS to pay for itself.

HS lasts 10 seconds but has an 8-second cooldown, meaning you can refresh it before it expires. In single-target situations, it's very rare for all 8 charges to be consumed, but it does happen on certain fast-hitting bosses. It's more common to use up all the charges in an AoE-tanking situation, but even with 3-4 mobs you won't often see HS get used up.

HS damage scales with both attack power and spell power. The amount of damage absorbed by a block scales with block value (obviously).

Consecration: AoE threat and damage. Affects all mobs within 8 yards of you for 8 seconds, with an 8-second cooldown. This is obviously a great tool for multi-target tanking, but it's also a decent single-target threat generator, and worth using if you have the mana for it.

Consecration will break most forms of crowd control (polymorph, sap, ice trap, etc) so take care with your positioning when consecrating around CC'd mobs.

Consecration continues to "tick" in the same location where you cast it. This can be useful for pre-consecrating an area that mobs will have to run through, and then moving elsewhere. The mobs will run through your consecration, take damage, and run over to you (assuming nobody else has built threat on them). However, it also means that once you've consecrated, you can't consecrate in another spot for 8 seconds.

The Glyph of Consecration increases its duration by 2 seconds, but also increases its cooldown by 2 seconds, so it still doesn't allow you to have multiple consecrations up at one time. The only real benefit from the glyph is that it makes consecration a bit more mana efficient (you only pay once every 10 seconds instead of once every 8 seconds). Most threat rotations rely on casting Consecration every 9 seconds, so this glyph is generally not taken by prot paladins.

Consecration scales with attack power and spell power.

Judgements (JoL, JoW, JoJ): Fairly low damage. The main point in judging a mob is generally not to build threat, but to keep up the JoL or JoW debuff for your group's benefit. Judgements are part of most standard rotations, but when non-standard threat abilities become available (e.g., Exorcism) Judgements are usually the first thing replaced. If other paladins in your group are keeping JoL and JoW up, you can feel free to ignore judging entirely if you like. (However, you may still need to judge to keep the Judgements of the Just effect active, if nobody else is providing a substitute effect, e.g., Thunderclap).

Healing done by JoL scales with attack power and spell power, while the mana restored by JoW is always 2% of the total mana of the recipient. Since both Ret and Holy paladins usually have higher AP+SP than Prot paladins, you should have other paladins do JoL if they're present, while you do JoW or even JoJ.

JoJ is rarely used in PvE, and virtually never in tanking. The only real use is for preventing trash mobs from "fleeing in fear", but there are many other tools for handling fleeing mobs, and mobs in raids almost never do this anyway.

Damage from judgements scales with attack power and spell power.

Seal of Vengeance/Corruption (SoV): This is generally considered the premier tanking seal, since a full stack deals more damage than SoR, and it can be kept active on multiple targets at the same time, especially with HotR.

The DoT will continue to tick for 15 seconds after the last time it was refreshed (melee or HotR hit). This can be very useful for mobs that have a threat-wiping ability. It also means that you don't lose any substantial dps/threat from the seal when your melee attacks are avoided. On the downside, the DoT nature of the damage means that Reckoning will have very little effect on SoV damage. (However, even if you do have Reckoning, SoV is still better for threat than SoR.)

The Glyph of Seal of Vengeance adds 10 expertise when SoV is active (-2.5% chance to be dodged or parried, so this means your chance to hit goes up 5%). This applies to melee attacks and to HotR (parries of HotR will show up as "deflected" but it's the same thing). Although not hitting doesn't cost you very much in threat when you're using SoV, getting parried still has the undesirable effect of speeding up your target's next autoattack.

Seal of Righteousness (SoR): In most situations this seal is less effective than SoV, but it can be useful for situations with fast-dying mobs that don't give you time to build a full SoV stack. Effectively, SoR sacrifices steady threat for "up-front" threat; you may prefer to start a fight with SoR to help establish threat quickly and then switch to SoV for the long haul.

The Glyph of Seal of Righteousness increases the damage done by the seal by 10%. While this is a nice effect for times when you're using SoR, you won't be using SoR often enough to make this worth using a major glyph slot.

Seal of Blood/of the Martyr (SoB): Rarely used for tanking due to low threat and the recoil effect. Can be handy for tanking instances you outgear since the extra mana from healing SoB recoil will be helpful.

Defensive and Utility Spells

Righteous Fury: See above for threat information. With talents, this reduces incoming damage by 6%.

Devotion Aura: At maximum rank and fully talented, this provides an additional 1808 armor to everyone in your group/raid within 40 yards. This is the staple tanking aura, and it helps your fellow tanks as well as yourself. You'll be using this in 95% of serious tanking situations.

Fire/Frost/Shadow Resistance Aura: These auras increase resistance to the specified elements by 130. These are invaluable on resistance-based fights, because they dramatically reduce the amount of resistance gear you (and others) need to wear.

Note that Shadow Resistance Aura has the same effect as the priest Shadow Resistance spell and the two do not stack, so you should only use this aura in groups with no priest.

Blessing of Kings: A 10% increase to all primary stats. This is useful for all classes/specs, but it's the most important blessing for a tank due to the stamina increase, which has the effect of increasing a tank's total health by around 8%.

Blessing of Sanctuary: Reduces damage taken by 3% and gives mana on a dodge/parry/block. The second effect really only applies to Prot paladins.

Note that the Discipline Priest talent Renewed Hope provides the same 3% damage reduction to the entire raid (with much less work) and the two do not stack. Thus, in groups with a Discipline priest, the only real beneficiary of this blessing is yourself or another Prot paladin, and then only if you can't maintain mana from other sources such as Spiritual Attunement and Divine Plea.

For tanks in "serious" tanking situations, this blessing is always inferior to Blessing of Kings, because the 8% health boost from BoK will improve survivability more than the 3% damage reduction from BoS. In fact, even for non-tanks, BoK will still improve survivability more than BoS.

Hence, BoS should be used on:

* Prot paladins who already have BoK, who have no Disc priest in the group, and who need the mana.
* Other tanks that already have BoK, who have no disc priest in the group.
* Other classes/specs that already have BoK and BoM/BoW as applicable.

Divine Plea: Restores mana but reduces healing done with Holy Light and Flash of Light. The Guarded by the Light talent causes this spell to refresh anytime you hit with an autoattack. The Glyph of Divine Plea reduces damage taken by 3%, and is strongly recommended for tanking. Properly managed, this spell can be kept active for most, if not all, of a fight, constantly providing mana and mitigation. In effect, this ability serves as a weak analog to a warrior's defensive stance, dramatically improving tanking ability while reducing healing power.

Sacred Shield: With the Divine Guardian talent, this spell lasts for one minute, and the shields procced last 12 seconds. At normal levels of spellpower for a tank, each shield will generally absorb somewhere between 1k and 2k damage. This is something you should be using on yourself or on another tank when you're tanking; while it's really not reliable mitigation, it's still a very mana-efficient reduction in the damage you take and will save your healers some mana if nothing else. Note that as of the 3.1 patch, Sacred Shields with different ranks of Divine Guardian can stack on one target, which means that frequently a prot and a holy paladin can stack their shields on the same target. Also as of 3.1, you can only have one Sacred Shield active at a time.

Hammer of Justice: With full talents this can have a cooldown as low as 20 seconds. While most bosses are not stunnable, many normal mobs are, and this can be invaluable in controlling multi-mob situations. This is especially effective with caster mobs; with good timing it's sometimes possible to force a caster mob to go 10 seconds or more in between damage dealing. Another point worth noting is that stunned mobs can't dodge, parry or block, so in some cases stunning a mob can dramatically increase the dps of your melee on it.

HoJ also interrupts spellcasting for 3 seconds, even on mobs that are immune to stuns. Generally this isn't long enough to let you be part of a serious interrupt-rotation, but it can be useful sometimes as an emergency interrupt, or for interrupting a mob you're off-tanking by yourself.

Seal of Justice: This isn't used terribly often, but situationally it can be useful for disrupting some caster mobs a bit. Keep in mind that HotR has a chance to proc this seal on each target, so throwing the hammer at a group of 3 casters has a good chance of interrupting one or two of them. In rare cases you can use SoJ along with judicious use of HoJ and the silencing effect of Avenger's Shield to keep a caster mob largely shut down.

Seal of Wisdom: Occasionally useful for regenerating mana when you can forgo the threat of SoV or SoR. SoW can proc on each bounce of HotR, so a full 3-hit HotR with SoW up is usually mana-profitable.

Seal of Light: In offtanking situations where you absolutely don't need to do any threat or damage this can give your healers a very very tiny bit of help.

Righteous Defense: In addition to casting this on a friendly target per the tooltip, you can also cast it on a hostile target, in which case it will take effect on that target's target. (This used to require a macro.) This means that usually you can use RD like a normal taunt and simply cast it on the mob you want to taunt, but be aware that if there are four or more enemies attacking the same friendly you're not guaranteed to get the one you're targeting.

Regardless of how you target this spell, the 40 yard range applies to the distance between you and the friendly target you're trying to taunt. If a mob next to you is targeting a friend who's 50 yards away from you, you will not be able to taunt the mob with RD. (This is a case where you use Hand of Reckoning instead.) On the other hand, if a mob 50 yards away is targeting a friend who's next to you, you can use RD to taunt the mob.

Righteous Defense shines in situations where one friendly is being attacked by multiple mobs, such as rescuing a healer from healing aggro, or saving an over-zealous AoE'ing warlock or mage. For best results, follow up RD with a threat-building move; Avenger's Shield often works well for this.

The main drawback to RD, aside from the sometimes-awkward range restrictions, is the fact that it will taunt up to three mobs whether you want them or not. One situation where this can be especially problematic is when you're trying to trade mobs with another tank, such as on the Four Horsemen encounter in Naxxramas. This is one of the main reasons Hand of Reckoning was added to the game.

The Glyph of Righteous Defense increases its chance to hit by 8%, making it much easier to reach the hit-cap with this spell (see the stat reference). This can often be useful in hectic situations, especially when juggling multiple mobs or large numbers of adds.

Hand of Reckoning: This functions like the standard single-target taunts of other classes, and additionally does a small amount of damage for purposes of pulling or breaking crowd control. Use this in situations where RD's drawbacks make it unusable or problematic, such as:

* switching mobs with another tank
* taunting one and only one mob out of a pack, such as on a pull
* taunting a mob that is targeting a player more than 40 yards from you, or not targeting a friendly target at all.

Divine Shield: Immunizes you to "all" damage for 12 seconds and clears "all" debuffs. (Quotation marks are used because there are in fact a number of effects in instances and raids which are not affected by Divine Shield.) This is generally not something you'll use while tanking, because it causes mobs to de-aggro you, but it can be useful sometimes when off-tanking, or during pauses in tanking, especially in conjunction with Divine Sacrifice.

Divine Protection: Reduces all incoming damage by 50% for 12 seconds. The benefit of this over Divine Shield is that it doesn't cause mobs to de-aggro. With talents, this has a 2-minute cooldown, so don't hesitate to use it when you think it's needed.

Cleanse: Theoretically removes up to three debuffs from a friendly target, one each of poison, disease, and magic. In practice you'll virtually never get a three-for-one cleanse and only rarely will you get two debuffs at once, but this is still a very powerful debuff-remover because you don't have to put any thought into which kind of debuff you're curing.

Because Cleanse is instant-cast, you can cast it while tanking without exposing yourself to extra damage, and unlike other "healing" spells it isn't hurt by Divine Plea. Hence if you've got a safe threat lead on the target(s) you're tanking, you can help out the raid/group's healer(s) a bit by throwing a few Cleanses.

Also worth noting is that Cleanse will only cast on a target that has a debuff; hence you can spam it on a target that you're expecting to receive a debuff with very little drawback. A good example of this is when you're tanking Steelbreaker in the Iron Council fight in Ulduar. Steelbreaker has a "Fusion Punch" ability that leaves a magic debuff on the tank. Since Fusion Punch has a cast time, you can watch Steelbreaker's cast bar and spam Cleanse on yourself as he's about to complete the cast, and remove the debuff only a fraction of a second after it's applied.

Hand of Freedom: Tanking often involves a lot of running around and repositioning; in fights with slowing or immobilizing debuffs, it can be very handy to be able to give yourself or someone else immunity to these effects for 10 seconds (or more, with talents). HoF will also remove existing effects.

A magic debuff that slows or immobilizes can be removed with either HoF or Cleanse. Cleanse has no cooldown, but HoF has the bonus effect of giving immunity.

Hand of Protection: This is generally not used for tanking, because like Divine Shield it causes mobs to de-aggro and switch targets. However it can occasionally be useful for clearing physical debuffs such as bleed effects. It can also sometimes be used to force a mob to switch targets in cases where a taunt has been resisted, but keep in mind that the mob will switch to whichever player is second on its threat list, which will not necessarily be you.

The more normal use for this spell is to protect non-tanks from attack, such as a healer or an AoE'ing caster.

Also note that HoP will neutralize any physical damage transferred by Divine Sacrifice. This can be useful in cases where the raid is taking large amounts of physical damage (XT-002 Deconstructor's Tantrum ability, for example.)

Hand of Salvation: This can be useful for cooling off the threat of a particularly strong dps'er, or for forcing another tank to lose threat in a fight where you're trying to hand off threat without taunting. A good threat-meter is essential for getting the most out of this spell.

With the Glyph of Hand of Salvation, this spell will reduce the damage you take by 20% when you cast it on yourself. This can be very useful in fights where you need another "cooldown" to reduce the damage you take, as long as you have a sufficient threat lead.

Hand of Sacrifice: Generally not something you use while tanking per se, since this spell redirects damage to you. However this spell can be very useful for supporting another tank, for "tanking by proxy" if someone else is being attacked. Also, in situations where you're taking little or no damage, drawing some damage to yourself can be helpful by giving you a chance to gain mana through Spiritual Attunement. Be careful of course, and be sure that someone's actually going to heal you.

Divine Sacrifice: Effectively like a Hand of Sacrifice on the entire raid, and like Hand of Sacrifice not something you'll often use while you're actually tanking. Damage-reduction or elimination effects will apply to the transferred damage, so if you use Divine Sacrifice and Divine Shield at the same time, all the damage transferred by Divine Sacrifice will effectively vanish and not affect anyone. In fact, if you have to use Divine Shield in a group, it's often a good idea to just go ahead and follow it with Divine Sacrifice unless there's a pressing need to do something else.

Be very careful when using this spell without an immunity or shielding effect, especially in 25-man raids. Raidwide damage effects can cause the damage from this spell to stream in very quickly. For example, XT-002 Deconstructor's Tympanic Tantrum ability damage each player in the raid for 10% of their total health every second. If your 25-man raid averages 20k hp with raid buffs, that's 50k damage per second; if you've got talent points in Divine Guardian, 20k of that damage will be transferred to you, in addition to melee damage transferred from the MT, etc. The only way to survive that without an immunity/mitigation effect would be to have healers ready to start casting big heals on you before the start of the tantrum.

This spell does not stack when used by multiple paladins. Most PvE-specced Ret paladins will have Divine Sacrifice, so if you have any in your raid make sure to coordinate with them to maximize the raid benefit.

Talents

At the level of difficulty this guide is aimed at (heroics and upwards), any serious tank will have at least 51 points in Protection -- quite often more -- and at least 6 points in Retribution for Deflection and 1/2 Improved Judgements. Beyond that, opinions vary. Most builds go with at least 53 points in Protection to pick up every talent in the sixth tier and deeper (except perhaps the second point in Spiritual Attunement), and then use the remaining points to fill in personal preferences or raid needs.

This spec represents the talents generally considered "required" for tanking, with the remaining 12 points being fairly flexible and subject to personal preferences or the needs of specific encounters.

Divinity: Opinions vary on how useful this talent will ultimately turn out to be. It's certainly an increase to the amount of healing your healers can do to you, but it's not clear how useful this will be (a lot depends on how much future content stresses healers). One thing that's clear is that this talent will be more effective for a main tank than an off-tank, since the MT generally has more healers assigned.

Obviously if you're forced to heal on some encounters, this talent has extra value. In theory, it should affect self-healing effects twice (since it's healing done by you to you) for a total of slightly more than 10% extra healing from healthstones, potions, bandages, Seal of Light, and Lay on Hands (which probably won't matter since LoH already heals you for 100% of your health.) (Poster qixxin has verified that this talent affects self-heals twice, including all of the above-listed effects.)

Divine Strength: This is a staple tanking talent; strength is a primary threat stat as well as a moderate defensive stat. Take this before Divinity; it belongs in any serious tanking build.

Stoicism: This is generally considered a PvP talent. Stuns do happen in PvE, but not frequently enough to merit this talent, and dispel effects are even rarer in PvE. Still, it may be useful on some specific encounters.

Guardian's Favor: Also a mostly-PvP talent. If you find yourself with Hand of Protection constantly on cooldown and wishing you could use it more often, then pick this up. Otherwise skip it.

Anticipation: This is a must-have tanking talent. Not only does dodging save you damage, it also restores mana through Blessing of Sanctuary. This talent does not suffer from diminishing returns and doesn't contribute to diminishing returns on dodge from gear: Your dodge chance will always be 5% higher with this talent than without it.

Improved Righteous Fury: Essentially required. You'll always have RF up when tanking, so this is basically a permanent 6% across-the-board damage reduction.

Toughness: Another must-have talent. A 10% boost to your armor value is roughly a 5% reduction in physical damage taken before blocking is considered, and effectively a larger reduction in post-blocking damage. The reduction in snare duration is also more useful than you might expect, since many mobs will use rooting or slowing effects.

Divine Sacrifice: Using this while tanking is usually not a good idea, since you don't want to increase the damage you're taking, but this can be a useful ability during off-tanking situations, or during phases of a fight in which you aren't tanking anything. It's especially useful in conjunction with Divine Shield, since the two abilities together simply cause all the absorbed damage to go away. Divine Sacrifice can only absorb a total of 150% of your health in damage before expiring. However it appears this is a limit on the damage dealt to you, not the damage saved on other people, so using this while Divine Shield is active will keep it up for its full duration no matter how much damage it sucks up. Also note that the damage transferred preserves its type (physical, holy, fire, etc) so you can resist it as normal, and if you have Divine Sacrifice and Hand of Protection active on yourself at the same time, physical damage absorbed will go away and not count towards "overloading" Divine Sacrifice.

Divine Guardian: The increase in effectiveness for Divine Sacrifice is nice, and if you use Divine Sacrifice with an immunity effect it represents a pure gain in the amount of damage you divert. As of the 3.1 patch, multiple Sacred Shields can be active on a single target if they have different levels of this talent. Since most Holy paladins don't have Divine Guardian, this talent allows you to stack your Sacred Shield with a Holy paladin's. The increase in duration applies to both the Sacred Shield spell itself (60s duration with the talent instead of 30) and to the shield effect it creates (12s instead of 6). A new shield "proc" while an old shield is active will simply refresh the effect, but will not stack. So in addition to making your Sacred Shield more powerful, it also reduces the busywork involved in keeping it active. The combined effects make this a very nice talent.

Improved Hammer of Justice: Judgements of the Just will already reduce your HoJ cooldown from 60 seconds to 40, so if you have that talent (which you should) this talent will further reduce the cooldown to 20 seconds. Whether this is useful depends a lot on your playstyle; some tanks love to use HoJ as a stun and an interrupt frequently on adds and trash, and some almost never use it. Basically, if you find HoJ is always on cooldown when you want to use it, this is the talent for you; otherwise spend the points on something else. (Personally I'm finding HoJ extremely useful in Ulduar in a number of situations.)

Improved Devotion Aura: Strongly recommended. Devotion Aura is the aura of choice for tanking things that hit hard, and the extra 600 armor at level 80 is not insignificant. Moreover, this helps other tanks as well, and the healing bonus helps healers no matter who they're healing. This is one of the talents that you'll generally be expected to bring to a raid.

Blessing of Sanctuary: Core tanking ability and required for other core tanking abilities. As of the 3.1 patch, this does not stack with the effect of the priest Renewed Hope talent (which also reduces incoming damage by 3% on the entire raid), so if you have a Discipline priest in the raid the only person who's really going to benefit from this blessing is you. This can be extremely annoying if you have 3 or fewer paladins in the raid.

Reckoning: This is a lot less useful than it was in TBC for three reasons. First, melee attacks and seals are a much smaller fraction of threat generation than they used to be. Second, now that SoV is the premier tanking seal, the extra swings have very little effect beyond the extra white damage. Third, one of the virtues of Reckoning in TBC was that it worked with two-handed weapons as well as one-handers, but WotLK prot paladins do far more dps with a one-hander and shield than they ever could with a two-hander.

Reckoning can still be handy for soloing/leveling/questing, where you'll be using SoR or SoW while fighting multiple mobs at a time, but it doesn't do much for tanking. And since it's easy to spend 25 talent points in the first five tiers on core tanking talents, it's not a useful point-soaking talent either.

Sacred Duty: Must-have. An 8% stamina increase for two talent points is a ridiculously good value. The cooldown reduction on the bubbles is just icing on the cake.

One-Hand Specialization: Strongly, strongly recommended. Since you'll always be using a one-hander and shield (see comments for Reckoning) this is a permanent 10% damage increase. It's true that threat generation is not difficult these days, but burst threat is still quite important. And if your dps is about half of what the dps'ers in your raid are putting out, then this is still equivalent to a 5% damage increase for one of them, which is nothing to sneeze at.

Spiritual Attunement: This talent provides mana that scales with the damage you take (provided you're getting healed, of course). With the new refreshing effect on Divine Plea, this talent is of debatable value. On the minus side, if you can keep DP up near-permanently you'll probably never notice this talent. On the plus side, keeping DP up permanently is one more thing you have to actively manage, whereas Spiritual Attunement is entirely passive so it can be a good value if you often find yourself too busy to pay attention to Divine Plea.

Holy Shield: Must-have, obviously. If you think you won't need Holy Shield for what you're doing, you're in the wrong tree to begin with.

Ardent Defender: Opinions differ on the value of this, but most serious tanks consider it well worth the points. Critics points out that the talent can be "leapfrogged" (that is, it only activates after the blow that takes you below 35%) and that if you're falling below the threshold regularly it's probably a sign that you should talk to your healers.

My view is that the proverbial shit happens; even the best healers are going to occasionally have to run out of a fire, or have a momentary lag hiccup, or get distracted by their spouses/children/pets, or whatever, and AD is potentially a lifesaver in those situations. Also, many bosses have burst damage abilities that are designed to push you down to low health, and AD helps you survive the next few small attacks before heals can land (consider Maexxna's web-wrap.) Considering that your death will frequently mean a raid wipe, I'd say this is a pretty important talent even if it's less than perfect, and a very good value for 3 talent points.

Redoubt: Must-have. This is worthwhile for the increased block value alone, but the proc effect can also be nice if you don't have enough avoidance to block all hits with Holy Shield alone, or if you're tanking multiple mobs and your Holy Shield charges are getting used up frequently.

Combat Expertise: Also a must-have. The extra stamina alone is worth the price of admission, but the expertise gives a little extra threat and mitigation, and even if you can't rely on crits for threat they're still fun. (Note that as of the 3.1 patch, this also affects spells.)

Touched by the Light: Another must-have. The spellpower from this talent adds rather substantially to your threat. The crit healing effect has nothing to do with tanking, obviously, but it's still nice for situations where you're forced to heal -- even if that means nothing more than soloing.

Avenger's Shield: Must-have.

Guarded by the Light: Must-have. This talent allows you to keep Divine Plea active permanently while tanking or dps'ing, and a 6% reduction in magic damage is nothing to sneeze at.

Shield of the Templar: Must-have. Ridiculously good for many reasons.

Judgements of the Just: The increase in SoJ stun duration is mostly a PvP effect, and the reduced HoJ cooldown is nice if you're a frequent HoJ user. However, the real importance of this talent is the slowing effect, which is a substitute for Thunderclap or the druid/DK equivalents. You'll want this effect up in some form anytime you're tanking something serious. Bosses have special attacks on cooldowns that aren't affected by this, so a 20% reduction in attack speed isn't a 20% reduction in damage taken, but it's still a significant benefit.

The question that gets asked frequently is whether there's any reason to take this talent if you know you're going to regularly have a prot warrior, feral druid, or frost DK in your raids. The answer (in my opinion, anyway) is yes. Raids frequently have to split up, with tanks in different areas where they can't debuff each other's mobs. The value of JotJ is that it's yours, and you will always have it on whatever you are tanking, even if your prot warrior is on the other side of the room. Unless you're customizing your spec for a specific fight where you know this won't be a problem, it's best just to take this.

Hammer of the Righteous: Must-have.

Retribution

Deflection: Must-have for tanking. Like Anticipation, this doesn't have anything to do with diminishing returns; it's always worth exactly 5%, and parries provide mana through Blessing of Sanctuary.

Benediction: All tanking spells are instant-cast, so this basically stretches your mana bar 10% further. Generally speaking, mana shouldn't be a significant enough issue when tanking to make this talent really necessary. If you've got 2/2 Spiritual Attunement and you're still frequently running low on mana, you might want to invest some points here, but otherwise it's probably not worth it.

Improved Judgement: One point is a must-have because the optimal ability rotations require a judgement every 9 seconds.. The second point is a matter of personal taste.

Heart of the Crusader: Not a tanking talent, but if you run with a smaller group and don't have a Ret paladin, this can be a nice dps boost for the raid.

Improved Blessing of Might: Also not a tanking talent. However, depending on the composition of your raid you may find yourself often casting BoM on melee dps and hunters. If so, this can be a nice dps boost for them.

Conviction: Not really necessary in any way, but big ShR crits can be a lot of fun if you have the points to blow. Also effects spells, so this can be handy if you end up healing frequently. Once your gear reaches the Naxx-25 level of better, this is worth (slightly) more dps per point than Seals of the Pure.

Pursuit of Justice: Very very nice to have if you can get it. Tanking frequently involves moving, repositioning mobs, dodging fire, etc, and a permanent run-speed bonus is a very nice thing. PvE mobs also often have disarm abilities, and the reduction in duration is nice since you can't HotR while disarmed. This talent doesn't stack with gear enchants that do the same things.

Sanctified Battle: A nice talent if you're this deep in Ret. One point in this is always better than one point in Conviction, since it does everything Conviction does and more.

Crusade: This talent is a flat 3% damage increase, and against many many raid mobs a 6% increase. Nice if you can get it.

Holy

Seals of the Pure: This is not a huge increase in damage, but it's not trivial either. Once you get to the Naxx25 gear level, this talent is slightly inferior to Conviction point-for-point. The only reason to get it is if you want to unlock the deeper tiers of Holy.

Unyielding Faith: Not a huge deal, but it's nice, and if you're here anyway with points to burn it's not a bad choice.

Aura Mastery: Note that the doubling effect doesn't apply to talents that improve auras; hence if you use it with Devotion Aura for example, you'll only get the base 1205 extra armor, even if you have the Improved Devotion Aura talent. Still handy as a kind of weak shieldwall that can also help other tanks.

Improved Lay on Hands: This is a very nice talent from a tanking perspective. With full talent points, it's a 50% buff to armor during the duration, which is roughly a 25% reduction in all physical damage taken. Effectively this turns LoH into a kind of stackable mini-shieldwall. With the appropriate glyph you can reduce the cooldown to 11 minutes, and it can be cast on other tanks as well. Potentially very useful if you find yourself in need of another damage-reduction or instant-healing cooldown.

Gearing for Survival

There are several ways to enhance your survivability. The first is to eliminate incoming critical hits; this is generally considered mandatory for serious tanking. Beyond that, a tank can focus on avoidance (increasing the chance for attacks to miss completely), mitigation (reducing the largest possible amount of damage that can be done to you), or soaking ability (stacking stamina). There are advantages and disadvantages to all these approaches, and different encounters will favor different survival strategies.

Eliminating critical hits

When you begin accumulating tanking gear, the greatest immediate danger facing you is critical hits. Critical hits from mobs do double the normal damage (spells cast by mobs cannot critically hit) and in most serious tanking situations taking a critical hit can be extremely dangerous, if not immediately fatal. ("Serious" here refers to tanking in level 80 heroic instances and raids. While leveling in non-heroic instances, you don't need to worry overly much about critical hits as long as your healer is on the ball.)

Because of this, the first gearing goal of any tank should be to become immune to critical hits. Becoming crit-immune against a level 83 mob, such as a raid boss, requires 540 defense skill. Assuming you're trained up to the level 80 maximum of 400 base defense, this requires a total of 689 defense rating from your gear. For heroic 5-mans, the highest level mobs present will be level 82. Preventing crits from them requires only 535 defense skill, or 664 defense rating.

(The crit-immunity threshold is sometimes referred to as the "defense cap" because further defense beyond that level no longer reduces your chance to take a crit. However, extra defense beyond this point will still add to your avoidance, and in fact defense is still a very good stat to use for that purpose.)

Becoming crit-immune is generally not something that happens "by itself" at pre-raid and early raid gear levels. You'll need to pay attention to your defense as you gear up, even after you've first reached crit immunity. In many cases, epic tanking gear has less defense rating on it than some blue pieces in the same slot, so it's frequently the case that upgrading a piece requires you to make changes in your gear elsewhere in order to stay above the crit-immunity threshold.

Avoidance

The philosophy behind gearing for avoidance is that the best way to minimize the damage you take is simply to keep it from hitting you entirely. Gearing for avoidance means increasing your miss, dodge, and parry chances as far as possible. A tank who's purely interested in avoidance doesn't make an effort to increase his block chance or block value; by his philosophy it's not worth removing a small bit of damage when you can focus on just avoiding the whole thing.

Avoidance tanks function best against bosses and mobs that land heavy blows, where block value isn't terribly significant and you'd rather just avoid the whole thing and let your healers cancel their big heals.

Gearing for avoidance places high value on defense rating, dodge rating, parry rating, and agility. Because of the high degree of reliance on dodge and parry, the avoidance-focused tank needs to be more concerned about the diminishing returns on dodge and parry than other tanks. According to Satrina on the TankSpot forums, the optimal path is to keep your ratio of defense rating to dodge rating between 1.5:1 and 2:1, and to never gem or gear for parry, except in cases where the item is an overall upgrade.

Mitigation

Like avoidance-gearing, mitigation-gearing also seeks to reduce the total damage taken, but it takes the opposite approach to that goal by focusing on reducing the size of the blows taken rather than their number. The first goal of mitigation gearing is to fill up the entire hit table with full or partial avoidance and thus eliminate any chance of an unblocked hit landing. (This is similar to the process of becoming immune to crushing blows from TBC.) Once that's achieved, all remaining gear freedom is focused on armor or block value.

Mitigation tanks function best against bosses or mobs that use lighter, fast attacks, where blocking can really shine. Mitigation gearing is also useful when first learning a fight, because it reduces the largest amount of damage you can take from one blow or from a multi-attack combination.

The valued gear stats for a mitigation approach are block rating and defense rating until the hit table is filled up, and then block value and armor after that point. Other avoidance stats (doge, parry, agility) are useful if they help fill up the hit table, but they're inferior to defense (which adds more total avoidance including block) and block rating (which is far more efficient per-point at filling the hit table. An addon like TankPoints is almost essential for this approach to determine exactly how much avoidance is needed. Additional avoidance beyond what's needed to fill the hit table is wasted.

Soaking

Gearing for soaking ability doesn't seek to reduce the amount of damage taken; rather, it increases the amount of damage you can take before dying. It's very rough on healers' mana, but it's hard to beat for overall survivability, especially on learning encounters. Soaking ability also has one major advantage over mitigation and avoidance, which is that it's useful against spell attacks, which can't be avoided or blocked.

Gearing for soaking ability is pretty simple: just stack stamina. More stamina is always better, and there's no diminishing returns on stamina, so there's nothing tricky or fancy here.

What's the best for me?

This depends on several factors, including the encounter you're tanking on, your personal preference and comfort, your healers' personal preference and comfort (which is almost always more important than yours), and the other tanks you're working with and what their gearing philosophies are.

This is one area where communication with your healers is critically important. Make sure to ask them regularly how hard you are to heal and why, and pay attention to what they say. If they say something like "Your health spikes a lot" then you might want to move away from avoidance and more towards mitigation. If they say something like "You're just taking damage too fast for me to keep up," then perhaps you should think about going the other way.

If you're in a guild or some other regular group that focuses on 10 or 25-man raids, talk to the other tanks about what they're doing. Different classes have different strengths and weaknesses, and it's to the advantage of your raid or your guild to have tanks that can handle different kinds of situations well. Paladins are strongest at mitigation tanking, due to the fact that Druids and Death Knights have no blocking abilities, and Holy Shield allows a prot paladin to block incoming blows with more consistency than a warrior.

However, this does not mean that every paladin should gear for mitigation. Instead, look at the other tanks you raid with. If you're in a 25-man raiding group and have tanks of each class, then the best move is to let each tank specialize in the kind of survival they're best suited for. On the other hand, if you're in a 10-man raiding group and the only other regular tank is a warrior, you don't have a class that's well-suited for soak-tanking, and in that case you should consider having at least one of you build a high-stamina set.

What's also important to remember is that these concepts are not mutually exclusive. You can take a middle-of-the-road approach that blends more than one philosophy (in fact, to some degree you have to, because you don't get to custom-design gear to fit your needs), and you can keep multiple sets of gear in your bags to switch for different fights (in fact, youre going to have to do this to some degree as well).

Gearing for threat

Aside from staying alive, the other main goal of a tank is to generate sufficient threat to keep mobs under control. In stable situations in Tier 7 content, this is generally considered fairly trivial. It's not hard to build a large threat lead over comparably-geared dps so long as you get to directly engage the mob the entire time. However, many fights will force you to suspend your threat generation on a mob or boss from time to time, even while they allow your dps (ranged dps especially) to continue hammering away. Furthermore, while sustained threat generation is generally not hard, it's often the case that your dps will want to open up as soon as possible, and a single missed Shield of Righteousness at the beginning of the fight can often have serious consequences. Thus, while most of your gearing effort will focus on survivability, you'll still have to think about threat.

Sustained threat

If you're interested in maximizing your raw threat generation over a long period of time, the best stat to focus on is Strength. With a normal tanking talent build, each point of strength on gear yields 2.3 attack power and approximately 0.75 shield block value, making Strength a better threat value then either attack power or block value alone.

After Strength, the next best stats for overall threat are hit and expertise (until you reach the cap for both), followed by block value, followed by attack power. Spell power is the weakest, because threat abilities that scale with spell power also scale with attack power at the same rate, and attack power is far cheaper from an itemization standpoint.

Burst threat

There are situations where being able to pick up an untouched mob (such as an add) is extremely important. In these situations, it pays to be able to guarantee a certain amount of threat right off the bat no matter what. By far the best way to do this is to stack enough hit rating to reach the hit cap. (The exact amount depends on your raid composition and buffs, but the short version is that if you have a draenei in your party you need +7% chance to hit, or 230 hit rating, and if you don't have a draenei in your party you need +8%, or 263 hit rating.)

Once you've reached the hit cap, you can guarantee that Shield of Righteousness, Avenger's Shield, and Judgements will always hit their target, because these abilities can't be parried, dodged, or blocked. This means you can guarantee generating a certain minimum level of threat on the target within a set number of global cooldowns, so you can tell your dps, for example, that they can go ahead and unload once they see the judgement hammer hit the target, or whatever specific signal you like. As an added bonus, once you've reached the hit cap, if you're using the Glyph of Righteous Defense, you can guarantee that your Righteous Defense will always work and won't miss or be resisted. (On mobs/bosses that can be taunted, naturally.)