Everyone has been in a group with a "bad tank". This is usually a guy who cant hold aggro. I challenge that this "bad" tank may not be the problem. It may be traced to "bad" DPS.
Since patch 3.0, WoW has been an AOE fest. AOE here there and everywhere. This is due to the fact that tank multi target tanking has become much easier (for Warrior especially).
This doesnt mean that every single pull should be an AOE pull. Not all tanks are created equal, but that doesnt make them "bad". I rarely have trouble keeping aggro on multiple mobs, but there are overzealous DPS that make is extremly difficult sometimes. The Ret Pally who judges on a mob after is was gun pulled. Heres your sign. The Warlock/Mage who uses SoC/Rain of Fire/Blizzard on a gun pull. Heres you sign.
Once a tank gets aggro, it is much harder to rip it off. If you pull it before he can get any kind of rotation down, it is much more difficult to get back (especially if it is multiple mobs). Holy Priest who purposely pulls with SwP or body pull on accident when the Warrior has not rage. Heres your sign.
If you run into a group where the tank is having these kinds of problems, take a step back and look at the big picture. DPS gear is scaling up up and away these days. Just remember, there is nothing wrong with focus firing a target down. That is how it was done since the beginning of WoW. If the tank is having trouble with big group pulls, try single targeting the target of the tank. If you are still pulling aggro, this may well be a "bad" tank.
On the same subject, a "bad" tank can be classified in many ways. Is this tank uncrittable? If the answer is no, he knows better and should not be tanking until he is (the obvious exception was when being 80 was new and they were still working towards gear). Is the tank sporting gear that is not gemmed? The he/she is "bad". Is the tank not paying attention to their surroundings and face pulling multiple mobs and causing wipes multiple times. The tank is probably "bad". A "bad" tanks is easy to spot. Some tanks just need a little help, which isnt bad.....its called team work.