This guide was written by Jasica/Ayasilkrose, Warrior Officer of Spaceballs the Guild on Draenor and 70 Feral Druid on Hyjal. It is reprinted here with the author's permission.
I don't know how much of this people already know, but I'm going to post a little about combat tables just so it's all here. What I'm about to say has direct implications for how you gear yourself for the end-game, or any time you fight mobs higher level than you.
A combat table is how WoW resolves the result of a physical attack. Every time you attack (or someone attacks you) the game builds a table showing all the possible results and how likely they are, and then rolls a virtual 100-sided die (actually it rolls a 10,000-sided die, but let's keep things simple for now). Suppose you attack a mob your own level that has a 5% block/parry/dodge chance, and you have a 20% crit chance. The combat table will look like this:
21-40: critical hit
41-100: regular hit
So the server builds this table, rolls a 100-sided die, and just checks the table to see what happened. If the server rolls a 45, your attack is a regular hit. If it's a 21, you crit, if the server rolls a 4, the mob dodges, and so on.
Notice something: crits and regular hits are checked for at the same time. Wow does not check to see if you hit, and then check to see if you crit. This makes sense from a coding standpoint, since the game has to make only one check per attack rather than two.
What really happens when the game builds a combat table is that it puts in all the "bad" things that could happen (e.g., if you're attacking, it puts in the dodge/miss/parry/block chance), and then fills in whatever's left with regular hit slots. Crit chance replaces regular hit slots. So suppose you're fighting a mob with a 50% total chance to dodge/parry/block/miss. That leaves a total of 50% chance to get a regular hit. If you have a 60% crit chance, your total chance to crit is ... that's right, 50%. Past 50%, there are no more regular hit slots on the table to be replaced.
A couple of things.
+crit and +hit are equivalent in most cases. Think about the combat table above. If you add +1% hit to your gear, you have turned a 1% miss slot into a 1% hit slot. Overall dps increase: +1% of your regular damage. If you add +1% crit to your gear instead of the +hit, what have you done? You've replaced a 1% hit slot with a 1% crit slot. You've gained a 1% chance to do 2x damage, but lost a 1% chance to do 1x damage. Overall dps increase: +1% of your regular damage.
So when deciding between +hit and +crit, most of the time all you really need to ask is whether you have any abilities that proc off of crits (like a fury warrior's Flurry) or whether your crits do an abnormal amount of damage (like a marksman hunter or a feral druid). If the answer is yes, take +crit over +hit. If the answer is no, take +hit if you have any commonly used abilities that can't crit (like sunder armor). Otherwise, personal preference - the total dps increase is the same either way.
It's very possible to have wasted crit chance by the end of the game. +hit and +crit are not necessarily equal when you have a very large crit chance. Any time you're fighting a mob higher level than you (specifically, any time the mob's defense skill is higher than your weapon skill, before +skill), you have a chance to land what is called "glancing blow." There is no way to reduce this chance (it used to be you could reduce it through +weapon skill, but not any more). By the time you're fighting mobs three levels higher than you, the glancing blow chance is 40%. Your combat table, before we replace regular hits with crits, now looks like this:
16-23 miss (a +3 level mob normally adds 3% to your miss chance)
24-63 glancing blow
64-100 regular hit
Notice that only 37% of the table is devoted to regular hits (this is your "crit cap"). This means that against a level +3 mob you can use a maximum of a 37% crit chance. If you want to use any more than that, you'll have to use +hit gear to convert miss slots into regular hit slots, which you can then overwrite with additional +crit.
Dual-wielders are in an even worse predicament. Without +hit gear, a dual-wielder's table against a level +3 mob looks like this:
16-42 miss (+3% for mob level, +19% for dual-wielding)
43-82 glancing blow
83-100 regular hit
There's only room for a 17% crit chance on this table. If the dual-wielder wants to use a higher crit chance, he'll have to pile on the +hit gear to turn more miss slots into hit slots, and then he'll be able to convert the hit slots into crit slots.
Hunters are in a good position for glancing blows, because glancing blows apply only to melee physical attacks. Thus, even against a +3 level mob, a hunter will have a large number of regular hit slots he can safely convert to crits.
The other side of the coin: crushing blows. Any mob three or more levels above you has a chance to land what is called a "crushing blow." This is a hit that does 150% of normal damage (as opposed to 200% for a crit). A level +3 mob has a 15% chance of landing a crushing blow. As I stated previously in my warrior thread, as of patch 2.0 +defense skill no longer defends against crushing blows. But a very high block chance can. Look at the table for a warrior with a 10% block chance, 5% miss/dodge/parry chance, who uses shield block (for +75% block) fighting a +3 mob, remembering we first fill up the table with results that are bad for the attacker:
The warrior has so many slots filled with bad results for the mob that there is no room for any good results - while shield block is up, the mob has a 0% chance to get a normal hit, 0% chance to get a crushing blow, and 0% chance to crit. Thus if your block chance is ridiculously high, you can push crushing blows right off the table.
A caveat. There is a theory among WoW number-crunchers that "yellow damage" (e.g., heroic strike, sinister strike) is immune to glancing blows in the same way that ranged damage is. This theory has apparently not been proven, but it seems to be correct. Thus if your crit chance is over your crit cap, you probably still get to use the extra crit chance for your special melee attacks.
That's all I really had to say about combat tables and how to avoid over-critting. If that's all you care about, stop reading now.
If you want to build your own combat table, here's how you do it:
Mob's defense skill = mob level x 5
Mob's weapon skill = mob level x 5
Using these five steps, you can build your own combat table using your specific profile against any mob scenario you like, and figure out exactly how much +crit and +hit you need to optimize your dps.