A tank's job is to protect the group. A big part of that is controlling the enemy. A big part of controlling the enemy is staying alive. Tanks have a lot of tools to stay alive, and mastering those is a major component of learning to play a tank. On the other hand, some of these tools are on long cooldowns, and on some encounters they are intended for use at specific moments in the fight. Furthermore, staying alive isn't the sole responsibility of the tank, because there will always be one or more healers present whose job it is to keep the tank alive. As such, staying alive can't be the only thing tanks have to focus on.
So, let's back up a moment to controlling the enemy. "Control" includes things like positioning the boss, or doing specific things at specific times, such as swapping with an off tank. It also includes making sure the boss doesn't attack anyone else. That's where threat generation comes into play.
If threat generation is too easy then the entire risk of the encounter drops. Newsflash: we don't actually want encounters to be easy. We want encounters to be fun, and for most players, that includes both rewards and risks.
We want tanks to care about the buttons they hit instead of just relying on auto-attacking to control their target. We don't necessarily want very complicated tank DPS rotations, because as I mentioned above, tanks do have other things to keep track of. But we want their combat abilities to be engaging. Good tanks should be those who control, survive, and generate sufficient threat.
On the other hand, when threat is too hard to maintain, it can be exasperating. Tanks get understandably frustrated when the game is asking them to do something but not giving them the tools to do it. The non-tanks in the group also become frustrated, because they feel throttled. It's one thing when overcoming the boss is challenging. It feels worse when you know that another player is standing in your way, keeping you from achieving your top performance.
We don't usually want DPS classes to have to stop attacking in order to keep from generating too much threat. We do want players to pay attention. We don't think it's too much to ask for DPS and healers to wait a couple of GCDs for the tank to get the enemy under control -- we're not asking for five stacks of Sunder Armor these days. What we really mean by proper threat management is knowing things like when it's time to go all out, when it's appropriate to use a threat-reducing cooldown, and most importantly, which is the right target to be attacking. I'm not trying to bash pugs here, but I am amazed at how often a nuker will pick a random target instead of the one being tanked, then blame the tank for not holding aggro (and then blame the healer when they die). In short, if threat is too easy, the game is boring. If threat is too difficult, the game is frustrating.
How then do you guys (and the developers!) know when threat is a problem? Here are some handy guidelines.
-- If a tank is trying to generate threat on a single target, and it runs off to kill a DPS class, that's a problem.
-- If a tank is trying to generate area threat on a group, and the tanked things are running off to kill healers, that's a problem.
-- If Vengeance falling off causes the tank to lose threat, that's a problem.
These problems can have a couple of causes. It could be a problem we caused, meaning that even an expertly-played tank has low threat generation because our numbers are undertuned. Or it could be that only an expertly-played tank can generate threat because you're asked to manage too many abilities. Or it could be that Vengeance is the only thing allowing you to generate enough threat because the size of the buff is masking low threat generation from your abilities. It could also be a problem you need to fix on your end: if someone is nuking or cleaving a random target on a group pull instead of assisting the tank, that's not the tank's fault.
Now, there are things we don't like about threat as a mechanic. It's fairly gamey as game mechanics go and we think there are probably better ways to communicate it to players. There are some mods that do a very credible job given the limited information we provide, but overall we'd like to present threat better since we're asking you to take it seriously in the PvE game.
-Greg "Ghostcrawler" Street is the lead systems designer for World of Warcraft and the skipper of a very nice ship where they serve cute sandwiches with the crust removed. And gin.