This article summarizes in one place what a tank needs to know about mitigation and gear.
The following stats are described as a unit, most of which have their own individual pages for more information:
The attack table
To begin, there are several different outcomes of any melee attack. These are miss, dodge, parry, block, hit, critical hit, and crushing blow. When leveling up to 80, or if you played during the Burning Crusade, you may have seen a crushing blow. Currently in the game, crushing blows are only possible if the mob is 4 levels or more higher than your current level. Effectively once you reach level 80, you will never have to worry about these again since even raid bosses are only level 83.
Note that glancing blows only apply to offensive attacks, so they will not be described further in this article.
For dungeons and raiding, the first mitigation stat a tank must worry about is hitting the defense cap, which now pushes critical hits off the combat table. Remember that critical hits are double the amount of a normal hit. For a level 80, the defense cap for level 83 bosses is 540 defense when unmodified by talents or glyphs. In a heroic mode dungeon, the bosses will only be level 82 so the cap is 535 defense.
Defense at level 80 is acquired at a rate of 4.92 defense rating. Assuming the level 80 have 400 defense skill, you need a minimum of 689 defense rating contributed from your gear to meet the 540 cap, again only 665 for heroic bosses. Notably, completely eliminates the defense cap for druid tanks. It is very possible to have a high dodge or parry rating and then lose it via a stun and then critted. So having enough defense is vital to a tank's survivability.
Once a tank becomes dedicated to raiding, any pvp gear will be replaced with proper tanking gear. However new tanks still running heroics, keep in mind that resilience also mitigates critical hits and stacks with defense. If using resilience alone, 460 resilience rating is needed to completely mitigate critical hits from a level 83 raid boss, but only 443 for a level 82 heroic dungeon boss. This is because a raid boss has an 5.6% to critically strike and 5.4% for heroic dungeon bosses. When a tank has a combination of resilience and defense rating, sum the amount of crit reduction from both. 123 defense rating (or 25 defense skill) is needed to reduce critical hit chance by 1%. 81.96 resilience rating also reduces crit chance by 1%.
Once a tank has is uncrittable, tanks now try to mitigate regular hits. For tanks that cannot wear shields, these attacks can only be partially mitigated by accumulating a large armor rating and can never be pushed off the combat table. For tanks that can wear a shield, regular hits can be pushed off the combat table with block rating.
For shield wearers, the next stat they should strive for is the block cap. Much alike the critical hit cap, hitting the block cap will remove regular hits off the combat table altogether (as defense removed critical hits off the table). When block capped, every attack you take will be mitigated in some way. Fully mitigated hits are misses, parries, and dodges, where the amount of damage taken is reduced all the way to zero. The remainder of blocked hits will be reduced based on armor rating and block value (not to be confused with block rating and block chance).
If the tank is level 80 and the mob is level 80, the playing field is even, and the block cap is 100% avoidance. But when the boss is level 82 or level 83, the mob gains an advantage and has an extra chance to land a hit. For each level the mob or boss is above you, he is 0.2% less likely to be dodged, parried, miss, or to be blocked when hitting in melee combat. For a level 82 boss (in a heroic dungeon), the block cap for the tank is 101.6% and for raids (because bosses are level 83) the cap is 102.4%. A macro for displaying combat table coverage is given on the CTC page, making it easy to check on it while swapping gear around.
Especially for tanks that can block, remember that not all mitigation (block rating, block value, parry rating, dodge rating, armor rating) is created equal. Even though you must have 102.4% total avoidance or higher to be block capped, the table still only has 100 points. Even at the minimum 102.4%, 2.4% will always be wasted in a sense. This wasted avoidance will always come from your chance to block, never from your chance to miss, parry, or dodge. This is because block is the lowest stat on the table.
Stacking block rating is good when leveling to hit the cap with less gear. However, as a geared tank, hitting the cap using more miss, parry, and dodge chance is desirable to take less damage overall. Again this is because a regular or blocked hit is only partially reduced by armor and block value, but a parried, dodged, or missed hit result in the damage going all the way to zero.
On the flip side of the coin, in terms of reaching the block cap, chance to block (from block rating, not block value) gives the most bang for your buck. For the sake of comparison, we'll temporarily ignore diminishing returns. For every 16.395 block rating, you gain 1% avoidance. The same is true for 39.348 dodge, 49.185 parry, or 122.95 defense rating. Defense rating is a little bit special because while doesn't provide very much total avoidance per rating point, it's the way to increase your chance to miss an incoming hit once you reach level 80 and max out your weapon skill. Parry is also special (remember that defense rating also increases parry just slightly) because a parried hit increases your attack speed temporarily, but a dodged hit does not.
Finally, to explain diminishing returns, commonly referred to as "DR". You'll notice on the character screen in several places there is the remark "before diminishing returns." What you see is not actually what you get, in particular if you have a lot of any one avoidance stat. For this reason, you may find the best gear balances your avoidance stats somewhat evenly, to avoid the diminishing returns as much as possible.
Thankfully, base stats, talents, and racials are not impacted by DR, only gear is. At first DR has little to no impact on the rate of return, but if you stack (for example) dodge and dodge only, you will eventually get very little additional dodge for each additional dodge rating. You would take less damage overall by replacing some of the dodge rating with another type of avoidance, defense, block, and parry rating.
To calculate DR exactly, you need to use your class specific coefficient for each avoidance stat. When calcuating remember that base stats and talented stats have no DR, agility and defense rating do give additional avoidance, but they are affected by DR as well. Block rating is impacted by DR, but since most tanks replace the majority of their block rating with parry, defense, and dodge, data is not available for the block rating DR coefficients.
Shield block value
Lastly for shield wielders, when you have accumulated lots of high quality avoidance from parry, dodge, and miss, you can still reduce your damage taken from stacking shield block value (and armor of course). Block value has no DR, unlike chance to block (from block rating). Block value is important for new tanks as well, since fewer hits will be missed, dodged, and parried.