Edit 11/7/07: This page is a little old and confusing. Updating it with the 2.3 changes and what not at some point.
Assumed Information
When I say defense by the way, I am talking about defense skill not defense rating. The same goes for block (block value increases the amount you block, block rating increases the frequency). Assume that all statistics on this page are calculated for a level 70 player with maxed out skills against a level 73 mob. This page is only relevant for white damage.
 Player Weapon Skill: Level * 5 = 350
 Player Defense Skill: Level * 5 = 350
 Boss Mob Weapon Skill: 365
 Boss Mob Defense Skill: 365
 Critical Hit: Against a player with a defense rating equal to the mob’s weapon skill (350 vs 350), the mob has a 5% chance to crit.
 Crushing Blow: Level difference * 5 = (73 – 70) * 5 = 15
The Attack Outcome Table
When a mob hits you there are 7 possible outcomes: Miss, parry, dodge & block. All of these items are mutually exclusive, meaning that you can either block OR parry OR take a crit. You can never block crits, parry crushing blows, etc. For the sake of organization, I’ve separated these outcomes into two categories:
Avoidance/Mitigation: Miss, parry, dodge & block – ways that you can mitigate or completely avoid attacks.
Actual Mob Hits: Crits, crushing blows, normal hits – types of attacks that you can receive if they are not avoided or mitigated.
The green items are good: Green outcomes that mean you have either avoided or reduced the damage of an attack. The red ones are bad: Red outcomes that mean you have either taken a hit from the mob that either does normal damage or increased damage. Ignore Critical Hits for now and assume that it’s at 0%. Crits are reduced via defense, (at 490 defense you are uncrittable) I will explain that below.
The table is calculated in this exact order. So when you make the green items larger, the red items on the bottom get “pushed off” of the table, starting with Normal Hits. When all of the Normal Hits get pushed off, Crushing Blows will start to get pushed off. Ultimately, your goal is to increase the green items to the point where there are no red left. That means that ALL attacks against you will either be misses, parries, dodges, or blocks.

Possible attack out comes (against lvl 70 player by lvl 73 mob) 
Avoidance/Mitigation 
Miss: Base 5% + 0.04% * (Your Defense – 365) 
Parry: (see your character stat window) 
Dodge: (see your character stat window) 
Block: (see your character stat window) 
Actual Mob Hits 
Critical Hit: 5% – (0.04% * (Player Defense – Mob Weapon Skill)) 
Crushing Blow: 15% 
Normal Hit: Whatever is left, 100 – (Miss + Parry + Dodge + Block + Crit + Crushing 
Attack Table Keynotes:
 The entire table must add up to exactly 100% at all times. So, if your total Avoidance/Mitigation is 70%, Actual Mob Hits will be 30%
 All items on this table are mutually exclusive, meaning that for example, a block can never be a critical hit and so on.
 Furthermore, the outcomes are calculated in this exact order: Miss, parry, dodge, block, critical hit, crushing blow, normal hit. So, even if you increase your Avoidance/Mitigation to 80% and decrease Actual Mob Hits to 20% (let’s just suppose that the mob has 0% to crit you), your chance to receive a crushing blow will still be 15%.
 In order to eliminate crushing blows completely off of the table, you will need to completely eliminate the mob’s chance for an unmitigated and unavoided hit.
 Your goal, as a raid tank, should be to increase your total Avoidance/Mitigation to 102.4%.
The 1Roll Theory
The current theory is that, the server generates a single random number, then compares it against the hit table to determine the attack outcome (there are literally tomes of speculation and theorizing on this topic, but that’s not what this guide is for). So with this information, we can construct an outcome table for our fictitious tank:
Timmy the Tank’s stats 
Timmy the Tank’s attack outcome table (server side) 
Miss: 10% 
0 to 9 
Parry: 20% 
10 to 29 
Dodge: 20% 
30 to 49 
Block: 20% 
50 to 69 
Crit: 5% 
70 to 74 
Crushing: 15% 
75 to 89 
Normal: 10% 
90 to 99 
Explanation: A (level 73) mob attacks Timmy, in doing so the server “rolls” one random number between 0 and 99 in order to determine how Timmy responds to the attack. If the server generates a 52 Timmy blocks, if the server generates a 78 Timmy receives a crushing blow, and so on.
Mitigating Critical Strikes
Every point of defense increases your critical strike mitigation by 0.04%. So, against a level 73 mob, 490 defense will render you immune to critical strikes. That would be: Base 350 + 15 (to compensate for the 3 level difference) + 125 (to compensate for the mob’s base 5% crit chance).
490 is also the CAP for defense in relation to critical strike mitigation (unless they raise the maximum mob level beyond 73). Tseric was quoted as saying, “At 490 defense, the math caps out. ” What this means is that: Once you achieve 490, Defense will not reduce critical strikes any further. It will though, still increase your miss, parry, dodge, and block chance.
Resilience also decreases crit chance as well as crit amount, but it’s a PvP stat…so don’t bother collecting it. Remember that Defense not only decreases your crit chance (to 490 at least), it also increases your avoidance/mitigation…resilience only affects crit. 490 Defense is also really pretty easy to achive in preraid gear.
Eliminating Crushing Blows
A crushing blow is a hit that is 150% of normal damage and can only be dealt by mobs (not by players). It occurs whenever a mob has 15 or more points in Base Weapon Skill over a player’s Base Defense (in other words, a level 73 mob vs a level 70 player :P).
So if you have skilled your character’s Base Defense out to 350 (base means 350 WITHOUT any gear and WITHOUT any talent bonuses), only a mob with a Weapon Skill of 365 will be able to hit you with a Crushing Blow; in other words, a mob of level 73.
Unlike Critical Strikes, there’s no real way of directly mitigating the percentage of Crushing Blows that a mob will deal. BUT, as explained above, since all outcomes on the hit table are mutually exclusive, you can completely eliminate crushing blows with 102.4% Block + Miss + dodge + Parry
Where Did The Extra 2.4% Come From?
For each level a mob has over you, 0.2% needs to be subtracted from Block, Parry, Dodge, and Miss. It’s basically comparing your base defense (350) to the boss’ base weapon skill (365) to my knowledge. Just think of it this way: Every point of defense that you have increases your miss, parry, dodge and block….it’s something similar in an opposite way.
All Burning Crusade boss mobs are at level 73, that is a 3 level difference. So you need to subtract 3*0.2% from your dodge, defense, parry and miss. That is a total of 4*3*0.2% = 2.4%.
Mitigating Normal Hits
Normal melee hits that is. Level 70 reduction formula: Reduction Percentage = (AC / (AC+ 10557.5)) * 100. Caps out at 75%