Basic Tank MechanicsQ: What exactly do tanks do, and how do they do it?
A: They take lots of hard hits without dying, and they allow no one else but themselves to be so hit.
So a tank's role breaks down into
survival and
threat. Threat is a lot less of an issue now than it used to be, so I think it is fair to say that a tank gears for survival first, then threat, then maybe dps. I realize that tank dps is getting to be a valid issue now that tanks can actually do above 200 dps, but no tank in a progression role has the luxury of gearing for damage (unless you tank with damage, as may be the case with druids). His dps is going to come from his stats, which naturally comes from better gear.
Q: How does a tank survive?
A: Two stats that tanks and ONLY TANKS care about -->
mitigation and
avoidance.
They are not the same thing. There seems to be quite a bit of confusion or general disagreement about the relative values of these two stats, so let's look at them more closely, since you will use them to evaluate the overall "goodness" of a tank.
Mitigation refers to damage mitigation; as in, the amount of damage you take when you get hit. It doesn't assume you'll miss, dodge, or parry the attack. It's not based on chance, or procs -- it's active all the time. Your damage mitigation comes from how much armor you have (it can also depend on your block value, but we'll get to that later). It also has nothing to do with how hard spells hit you; it has to do with physical damage only.
Avoidance is your chance to avoid damage completely. It is the base miss chance of the attacker, coupled with your block/dodge/parry. These days, most people count block only as a mitigation talent (more on this later). Generally speaking, a tank's avoidance is his miss/dodge/parry. Again, this has nothing to do with avoiding spell damage, that's just school-based resistance.
Then there is the matter of
Effective Health. This is basically how much damage something has to do to you to kill you, and it directly relates to your armor and stamina. Let's assume you're offtanking Patchwerk and you eat a Hateful Strike -- you don't dodge, block, or parry it, it hits you squarely on the face. All that's saving you from death is your effective health -- part of that hit was mitigated by armor, and the rest was soaked up by your Hit Points.
There's also
defense. Is defense a mitigation or an avoidance stat? It depends -- under the defense cap, more defense will be a mitigation stat because it will lessen the chance a mob will crit you for 200% of normal damage. That's very bad, in case anyone was wondering. Above the cap, defense adds additional block/dodge/parry, so it is then an avoidance stat.
Finally, there's
block rating and
block value. I said earlier that most consider block to be a mitigation stat only. The reason for this is that unless your block value is high enough, you won't block all of the damage of the attack, you just block a part of it -- in which case, you've blocked (or mitigated) part of the damage. You can think of block rating as being an avoidance stat and block value being a mitigation stat, but most people just consider block rating and value to be both mitigation because you won't block all of the damage of a boss attack in most cases. What this means, additionally, is that once a tank becomes
block-capped (he has 102.4% avoidance, which we'll cover later also), that his block rating becomes effective health, because every attack that gets through will be at least partially blocked. That part that you blocked relates to your block rating, and it equates to more damage that you can absorb.
Q: What is a block cap?
A: When a mob tries to hit a player, the damage done is the result of
one dice roll (0-100) based on the following attack table:
Miss
Dodge
Parry
Block
Crit
Crush (not possible at level 80)
Hit
While it is one roll, the values of all of these are summed up in descending order, so that if your tank's combined miss + dodge + parry + block is above 100%, there's no room on the table for a crit, a crush, or even a normal hit. Every hit was at least partially blocked.
In other words, when you roll a dice between 0 and 100, it lands somewhere between 0 and 100. When you sum up the values on that table in descending order, if you find that the combined percentages of your miss/dodge/parry/block is over 100, that means that the value of the dice roll always falls within the realm of miss+dodge+parry+block. Everything else has been
pushed off the table. That's the block cap. That's what used to be called the uncrushable cap in TBC.
For example, let's say your miss+dodge+parry+block is 100, your miss is 10%, your dodge is 20%, your parry is 20%, and the 50% left is block. That means 0-10 is miss, 10-30 is dodge, 30-50 is parry, and the rest is block. The dice rolls 43, and that falls within the parry percentage, so you parried that attack. The next attack is 89, and that falls within block, so you block that attack. Probably not the entire hit, though...how much of it you blocked has to do with your block value.
How Tank Stats Relate to Tanks
Well, not all of these stats relate to all tanks, for example. Druids can't block (they don't wear shields) and they can't parry (they don't tank with weapons, although a giant bear with a mace in their mouth might actually be really scary). Their mitigation comes solely from armor because of the inability to block. Their avoidance comes mostly from dodge and defense.
Death Knights also don't block. They make up for it by having armor that scales higher than for warriors and paladins.
Druids can effectively get 540 defense just from talents alone. It doesn't mean that defense doesn't help druids, it just means that they don't need it to be uncrittable.
What is better, avoidance or mitigation or effective health or defense?
It depends.
You need to be defense capped, first and foremost. It's not hard. Do everyone a favor and don't go anywhere without being defense capped. You're not a tank until you are, you're just a liability.
If you are defense capped but cannot get block capped, it makes sense to gear/enchant/gem for more effective health (more mitigation and block) perhaps over avoidance. Higher-end tanking gear has avoidance on it, luckily it also has more effective health as a general rule.