This was written by Jasica/Ayasilkrose, Warrior Officer of "Spaceballs The Guild" on Draenor and 70 Druid on Hyjal. It is reprinted here with the author's permission.
A lot of this is probably stuff that people already know, so bear with me if I include something obvious just for the sake of completeness. This guide serves two purposes: first, to serve as a comprehensive resource for the theory of tanking; and second, to share what I've learned about protection tanking in particular.
The first question we should ask when approaching the theory of tanking is to ask why the tank role exists at all. Everybody knows that MMORPGs are built around three basic party roles: healers, damage-per-second dealers and tanks. But why have those three roles in the first place? Here's a simple way of putting the question: what can a party of four dps classes and a tank do that five dps classes can't?
The answer is, it depends. In some situations, as we all know from experience, tanks are superfluous. If five mages can put out enough damage to kill every mob before the mobs ever reach the mages, it's obviously superfluous to have a tank along for the ride. In fact, it's worse than superfluous, because four mages and a tank might not have enough dps to kill every mob before it gets into range.
Of course, most of the time (at least in instances) a party doesn't have enough dps to kill every mob before it gets into range. In that case, everybody's instinct is to have somebody tank the mobs. But why? Here's the only good reason: because it makes the mobs die faster while best preserving the party's resources.
How? Think about what would happen if there were no tank. The mobs would reach the mages, and... then what? The mages might pop frost nova, or polymorph, or blink or something. It doesn't really matter what specifically. The point is, they would stop killing the mobs in order to defend themselves (or else just die, which will certainly make the mobs die slower). Every point of mana spent on frost nova, every second spent running away to get distance, is one point of mana or one second not spent killing the mobs. If the mobs were being effectively tanked (we'll get back to how to tank effectively), the mages would be free to spend every single second handing out damage.
Before we go any further, we need to address what is probably the single biggest tanking myth: the idea that tanking is fundamentally about taking hits. That is Not True. Think about it: how does the tank getting hit help the mages kill things faster? It doesn't. But, you say, the tank has to be hit in order to let the mages do their job. Yes, that's true – but think about what you just said. Tanking is about making others free to do their jobs. It is not about taking hits. It is not about staying alive. Taking hits and staying alive are tools that a tank uses to accomplish his mission, but his mission is to let everybody else in the party do their job without worrying about some mob eating their face. That is the tank's creed.
The question now becomes how a tank goes about accomplishing his mission of keeping everybody else free to do their jobs. Fortunately, there is a very simple answer to this question. To keep the dps'ers free to dps and the healers free to heal, all a tank needs to do is two things: a). make sure the dps'ers have to do an absolute minimum of holding back, and b). make sure that nobody but the tank is ever being hit. And even more fortunately, both of those two things really boil down to one thing: generate as much threat as possible.
Before we go any further I should define some terms so we're all on the same page. Threat is a value that determines who a mob is attacking at any given time. Everybody in the party has a separate threat level for every mob in the fight. A few more definitions:
For purposes of this discussion there are only three things you can do to generate threat:
Also, let's clear up a possible misconception about threat:
Threat never decays. Once you've built up threat on the mob, it will be there until you're dead, the mob's dead, you use a negative threat attack like feint, or the mob wipes threat itself (what they call an "aggro wipe" – something only a few mobs, mostly bosses, can do).
This is true even if some other part of the threat attack went away. For instance, sunder armor is a common pure threat attack. It places a debuff on the mob reducing its armor. Sunder armor, like all such attacks, has two components: the debuff, and the threat. They are not connected. If the debuff wears off, the threat remains.
Threat is the key to understanding how to pull aggro from somebody. The threat rules are very simple:
Those are the only three rules there are for describing how a mob decides who to attack. We are now ready to understand why a tank's job is to generate as much threat as possible.
Imagine you're tanking some mob, and you're generating 10 threat per second. Now imagine that you're being assisted by a rogue who can generate 11 threat per second if he goes all-out on his dps. What's going to happen? If the rogue goes all out, he's going to pull aggro according to the 10% rule. Then he's going to have to defend himself (maybe by feinting, maybe by popping evasion), or the tank is going to have to taunt the mob – after which the rogue will pull aggro again. Alternatively, the rogue can hold back on his dps. Let us be clear here: that means the rogue's gear is going to waste. He's painstakingly ground out the weapons and armor necessary to generate 11 tps, but he has to hold back to a mere 10 tps. When he groups with you, he could get the same results by not holding back using gear that was a lot easier to get.
My point is not that it's always the tank's fault if he loses aggro. Sometimes your assists are several tiers above you, and there's nothing you can do about that. My point is that the tank sets the party's dps cap. The more threat the tank kicks out, the more dps the party can kick out. A tank should strive to produce as much threat as possible, both in terms of the gear he acquires and in terms of how he fights.
But what about staying alive? What about 440 defense and 315 fire resistance and armor and hit points? Isn't there more to tanking than maximizing threat?
Yes and no. Remember that survivability doesn't generate threat. Since the tank is the one getting hit, it's important that the tank have more damage mitigation than anybody else in the party. Otherwise you're spending the party's hit points (and the healer's mana) inefficiently. But how much more damage mitigation does the tank need? As much as he needs to stay alive through the entire fight.
It's important also to keep in mind that everybody in the party increases the tank's survivability. This is a fairly obvious point but it's worth emphasizing. There are four factors that contribute to how long a tank (or anybody) will live in a fight:
Note that the mob is responsible for how much dps it deals, the tank is responsible for how much dps gets through, the dps'ers are responsible for how long the mob is dps'ing, and the healer is responsible for how big the tank's HP "battery" is. In other words, how survivable the tank is depends on a lot more than just the tank. On the other hand, how much the dps'ers can dps without pulling aggro or behaving inefficiently and how much the healer can heal without pulling aggro or behaving inefficiently depend only on the tank.