The outcomes on the attack table used in WoW (refer to my earlier post about Blizzard's table-based combat system here) are, in order of dominance: miss, dodge, parry, block, critical hit, crushing blow, and lastly, normal hit.
The order of dominance is at the center of the table-based combat system. Simply put, the chance of any outcome in the table occuring depends on the cumulative chance of the more-dominant outcomes (those listed earlier in the attack table).
A couple of examples should clear things up. Being the least dominant outcome, the chance of a normal hit is dependent on the cumulative chance of everything else. If these chances add up to a 100% (which is not unreasonable), the chance for a normal hit will drop to 0%. If a very undergeared tank (with dodge, parry and block chances of only 5% each) is fighting a raid boss (~5% chance to miss and crit, 15% chance to crush), he will be receiving normal hits 60% of the time. The hits are not bad, per se, but the crits and crushes will make short work of him.
Now, let's say we have a tank with incredible amounts of defense, affording him a tank a 40% chance to be missed, and to dodge, parry and block. Miss, being the most dominant outcome, will happen 40% of the time. Dodge, the next dominant, will 40% of the time. That only leaves 20% of attacks that are parried. The rest of the parry chance is wasted. No attacks will get blocked either. Of course, with this kind of mitigation, the tank will have a very hard getting rage (mana, if its a paladin), and not be able to hold aggro. This time, the mob will make short work of the rest of the group. But such a scenario is too hypothetical to even consider.
Critical hits and crushing blows are very bad. Nuff said. The mob's weapon skill, which is always 5 times the mob's level, and the player's defense skill, which can be up to 5 times the player's level before bonuses from gear, affect the chances for these outcomes.
All mobs have a 5% base chance to score a critical hit (2x damage) on a player with the same defense skill as the mob's weapon skill. Each point of difference increases or reduces the chance by 0.04%, depending on which is higher. Raid bosses, considered level 73, have a 5.6% chance to crit a level 70 player with 350 defense skill. However, by increasing their defense skill by a further 140 (5.6/0.04), the player can effectively become immune to being crit. This 490 defense is generally considered the magic number that tanks aim to get. It also increases the chance to dodge, block and parry by 5.6% each. Note: The boss has its miss chance lowered from the base 5% to 4.4% by virtue of being level 73, and 490 defense increases it by a further 5.6% to give a total miss chance of 10%.
Ok, so crits are out of the way, rather easily, granting us a whole lot of extra avoidance in the process. Now for crushing blows.
A mob can only score a crushing blow (1.5x damage) on a player if it there is a difference of 15 between their weapon skill and the player's defense skill. A weaker form of crits, you say? The difference is that the chance to receive a crushing blow is always 15% as long as your defense is at the base cap (player level x 5), and increasing defense past this cap with talents or gear has no effect on this chance.
To eliminate crushing blows from the attack table, the player has to increase their combined chance to miss, dodge, parry and block to more than 100% (102.4% to be specific, to account for the level difference of a raid boss), literally pushing crushing blows (along with crits and normal hits) off the table, as the other outcomes are more dominant. The warrior ability Shield Block and the paladin ability Holy Shield help in this regard by substantially increasing their respective chances to block by 75% and 30(or 35)%. In fact, the huge boost from Shield Block almost trivializes the need for avoidance stats on gear, allowing warriors to focus on stamina instead.
Then why bother getting 490 defense to eliminate crits when it can be pushed off with high enough mitigation anyway? Primarily because Shield Block and Holy Shield have charges that might get used up before the skill has cooled down. This is especially likely when fighting more than one mob. Of course, the increased defense also has the secondary benefit of helping you get that much closer to the 102.4% mark by increasing your other avoidance stats.
Contrary to the example I used above of having 100% avoidance and having difficutly getting rage (or mana), the main outcome occuring here is block. Block is more of a mitigation stat, than an avoidance stat. Given the amount most mobs hit for, and the amount that gets blocked, there is still plenty of damage being taken, preventing said problem.