A bit of background, I played a mage in vanila and in BC. Tanking was someone elses problem, and healing was nice and all but couldn't really be relied upon. Mages were then, and are now, close to the bottom of the priority list when it comes to heals.
It wasn't until I reactivated and upgraded to WotLK that I even played a melee class in WoW. I was uninspired by my mage. Sure he has great lv 70 raiding gear, but I still almost died on the first random mob I encountered in northrend. I may have had my action bars in a mess. I might not have been able to find blink or frost nova. I might have neglected to spend any talent points post-patch.
In short, I spent 7 months playing Age of Conan, Warhammer Online, and of course my beloved DotA. In AoC I don't even remember what class I played, it was a pretty bad game. Warhammer had much more potential and in warhammer I played a tank. A chosen. A badass, plate wearing, proper tank.
And it was FUN.
None of this getting one shot by mobs. None of this oh shit please dont hit me please dont hit me snare kite run nuke snare kite run nuke fiasco that mages go through all the time.
Oh no... It's more like GET OVER HERE AND HIT ME YOU BIG WIMPY BOSS! Leave my squishies alone! Cmon hit me like you mean it ya big girly monster, etc etc.
Totally more fun than wearing a dress.
I know about bad tanks, I've been one, and I dare say it takes one to know one. I definitely had no idea if the tanks in my vanilla and BC raids were any good. They must have been because my guilds progressed, but all I know is that they didn't die a lot.
The thing is, a well geared poorly played tank can still be kept up by a skilled healer, let alone a bunch of them sprouting the latest healing epics.
So the people most likely to notice that the tank isn't much chop, are likely to be the healers, or the other tank, if he is paying attention.
So, for the rest of us, heres my list of ways to spot a bad tank. Some of these things could be caused by the encounter or supporting players not doing their job, but not all. I'm saying dont write your tank off if you see one or two of these things on occasion, but if you see lots of them all the time... something isn't right.
1. The Bad Pull.
Everyone knows this one. You see the pat incoming but the tank pulls anyway. Complete failure to control and / position the casters. Pulling more than one pack and causing a wipe, pulling when people aren't ready, pulling when people are dead, pulling when people are out of range, pulling when healers are out of mana...
2. The Bad Gears.
Harder to spot but with a little investigation anyone can come to know how tanks should be gemmed / enchanted / geared. Is your DK tank packing parry gems and agility enchants? Is your pally under the defense cap? Does your warrior think he can tank with a two hander?
This can be hard to spot if the DPS isn't enough to make it an issue, or the DPS classes are skilled enough that they watch threat and adjust their output accordingly. Essentially a tank who threat caps a DPS is generally doing it wrong. Tanks have some abilities which have a +threat modifier and as a rule of thumb they should spam these when possible. Lots of them are only available after a block or a parry or something, so the tank needs to be on top of his game to get full use out of them. If used correctly threat capping will only happen when the DPS significantly outgears the tank.
Once again hard to spot, how are you supposed to know if the tank is popping his anti magic shield at the right time to mitigate sarths next fire breath? A tricky one. I know when I'm doing it right, my health stays higher, and takes less scary spikes than when I do it wrong. If your tank's health is like a yoyo on crack, this might be an issue.
5. The Bad... dealing with multiple mobs.
The thing about lots of mobs is that there are lots of them, and the buggers rarely stand where you want them to. Some adjustment is usually required to make it so that the whole pack is standing in your aoe, and not hitting you in the back. Tanks turning their back on mobs often die a sudden death. Sometimes you simply have no choice, say your taunts are all on CD and a mob is beating on your healer. You may have to run over there and save the day, and hope you dont take too much damage from the mobs chasing you. Protip: use strafe at every opportunity to keep your back safe.
It is up to the tank to set the pace of any instance or raid. Is he taking waaay too long? Is it moving too fast for the rest of the team to keep up?
PS: A DK who pulls when in unholy or blood presence, or any tank who pulls in his DPS gear without realising... may not be a fail tank, as such, but they may be having a fail moment. Give them a second chance, its not the kind of mistake you make twice in one night...
Yes... I've done it plenty of times!