Getting over the fear
Fear is perhaps one of the largest driving reasons that raid leaders have against dealing with a tank who is underperforming. Tanks aren't as readily available and a bit more difficult to replace than a DPSer or healer, when one falls out of sight. For this reason, I've seen a number of raid leaders who are willing to let failures on the part of a tank slide far more than they would for any other player.
This is the biggest no-no that you could ever do. The first part of dealing with any poor tank is to understand that they are no different than any other player. They are not special, your raid does not rest exclusively upon their shoulders, and you will not completely fall apart should they leave. If anything, keeping around a tank who isn't performing up to par is far worse than keeping any other player. A poor tank is going to cause an encounter to fall apart faster than someone in any other role. If they don't know how to compensate for the mechanics at hand, then you'll quickly find your raid group smashing their heads against a brick wall. That's not a situation you want.
Tanks too can sometimes gain an inflated sense of self-importance for the health of a raid. I've been in a position where a raid tank actually told me that the raid wouldn't exist without him, that I had no choice but to keep him around because we could collapse if he were ever gone. Needless to say, he was promptly removed. Magically, the raid went on. Tanks aren't special; they are replaceable, and you will not end your world without them. There are always other tanks out there, and there are always immediate options that you can pursue to replace them until you can recruit another. Never let a tank dictate how you run a raid because they feel that you could not go on without them. You can.
The truth about threat
Despite all of Blizzard's efforts, threat is still an important part of any encounter that you run. While threat on most boss mobs is never going to be an issue, and it hasn't been since this expansion began, there are always situations where poor threat generation is going to result in multiple deaths. Your tank absolutely must know how to pick up adds, and failing to do so is always just as much their fault as it is anyone else's.
Placing the blame on others is usually the first fall-back of threat issues for a tank. Let's get one thing clear: Threat is just as much the responsibility of the tank as it is the DPS. If you have mages or rogues who have the strange urge to throw out their strongest attacks before a tank has even managed to pick up a mob, then, yes, you should correct the issue with those DPSers; they need to understand that threat isn't an instant glue and that tanks do need time. On the same turn, though, tanks need to also be made to understand that they have to be doing everything in their power to get as much threat as quickly as possible.
I've seen it time and time again on encounter after encounter. An add will spawn; the tank will get in position and merely taunt the add, then sit there waiting for the mob to slowly meander over to them. This is wrong on so many levels, and in this situation, any threat issues you have are going to be the direct result of the tank's failing to do the job. First, healers can just as easily pull threat as a DPSer would in this situation. Taunt doesn't generate any threat; it merely gives the tank more than whatever the current top value was. Should the mob take its sweet time getting over to the tank, and mobs always do, it is easy for that mob to change pace to chase down a healer instead.
Further, DPSers don't always have the same luxury of waiting as tanks would like. While the DPS need not unleash their full force upon every new, spawning add, many of them are going to deal secondary damage up front -- DOTs, cleaves, AOE damage, there's tons of light damage that's going to be applied. A mere taunt isn't going to have the mob stick to your tank from this type of damage, and it isn't the DPS's fault when they get aggro. A tank needs to be on a mob the moment that it spawns. Taunting it is the right course of action, but they need to travel to the mob just as much as the mob needs to get to them. If DOTs are pulling threat, it isn't the DPS's fault -- it's the tank's.
When you're having threat issues, while you need to address the DPS that are causing problems, you similarly need to get on the tank to step up their game. In threat situations, everyone is to blame, tank included.
Location, location, location
Positioning is one of the most difficult tasks that a tank will ever have to deal with. Not only is the tank getting smacked on by a gigantic creature of some form that is probably taking up a majority of their screen, they also cannot stand in fire, nor can they position the boss in such a way that the melee DPS are in danger. It's a tall order for anyone, but it is a part of the tank's job. Taking avoidable damage as a tank is far more dangerous than it is for any other raid member.
If a DPSer stands in something terrible for a second or two longer than they should have, it generally isn't the end of the world. They'll require more healing than they should, but overall their life probably isn't in danger. For a tank, that's just not the case. Taking excessive avoidable damage puts them in a huge amount of risk; they also have a boss swinging at their face, which is enough damage on its own without adding to it. Tanks who fail to move out of damage fields quickly enough are going to cause issues, and you cannot allow this to happen. A tank who is taking more damage than they should is a liability.
There are numerous addons available that will provide warnings for players to move; tanks need to heed these warnings just as anyone else. If a tank can't seem to manage this, then they have to be replaced -- not permanently, but at least for that encounter. Have them switch to a DPS spec and practice dodging out of whatever it is that's killing them. Once they have it done while a massive monster isn't beating their face in, they can try it with the added danger.
Second, tanks cannot just live for themselves. They have to be aware of the positioning of melee. Getting yourself out of a fire only to have the entire back of the boss sitting in one helps no one. Encounters rely just as much on the DPS performing their job as it does for a tank to do theirs. Failures here are possibly more directed at communication. If boss positioning is an issue, try having one of your melee players call out when the boss needs to be moved. It can get irritating to the tank to have to be told what to do at times, but it's something that must be done.
It isn't just a job for the DPS
Make no mistake, tanks generally have the worst of the worst when it comes to encounters. They have to worry about threat, their cooldowns, boss positioning, adds spawning, all sorts of random crap. It can get confusing and overwhelming at times. That being said, a tank can't always have the luxury of passing off everything they don't want to deal with onto other players. I've seen it numerous times: Tanks refuse to perform some encounter mechanic or won't interrupt a spell because "it's the DPS's job."
Hate to burst your bubble, but no, it's everyone's responsibility to perform the requirements of an encounter. Tanks have interrupts just the same as many DPS do, and they have the same requirement of using them as well. Ideally, sure, you want to relegate that job to your damagers, but that isn't always an option, particularly in 10-man raids. Tanks who refuse to use their entire toolkit of abilities are just as wrong as DPS who refuse to do the same because it might result in lowering their damage.
A tank's job is to survive, and surviving often involves doing things other than standing toe-to-toe with the big bad. Interrupts, running to something, activating whatever, taking some debuff, or whatever the encounter might require rests just as much on their shoulders as it does anyone else's. If a tank is unwilling to do the task they are given, then handle them as you would any other player than won't do as their told -- remove them.
Tanks are a bit more tricky to deal with than other players, but at the end of the day, you really must view them as any other part of the raid group. You can never allow a tank to get away with something that you wouldn't allow a DPS or healer to do. That is perhaps a greatest danger than a poor tank is to a raid. Special treatment of one leads to resentment from the many. Players are people and should be treated with the respect that is due to them, but they are not granted anything more than that. Remember this.
Ready Check shares all the strategies and inside information you need to take your raiding to the next level. Be sure to look up our strategy guides to Cataclysm's 5-man instances, and for more healer-centric advice, visit Raid Rx.