5. Mobs everywhere, HALP!
A common issue for new tanks and healers alike are that they get paralyzed when too much things are happening at once. Usually it is better to do something, anything really (even /dance for the comic relief), than to just stare in panic as shit hits the fan. And because of how healing and tanking work, I can only recommend you to go dps if you actually feel like you have trouble to improvise, because that is much what healing and tanking is about (and what makes it so much fun imo!). One of the things I love about rerolling a character is that I get a chance to thoroughly get a feel for each skill that the class and role has at their disposal. What I often see people do "wrong" however is that they simply respec a character at max level, which makes it really difficult to find out when to use which skill and how in any other way than the hard way - by failing in pugs (which usually has people give up or not try at all). Most tanks have so much possibilities that aren't been put to good use because the tanks don't think about them or just don't dare to use them because they're not comfortable in their rotation. Heroic Leap for warriors, Hand of Salvation/Protection for paladins, interrupting for all tank classes to mention some very few. In lowbie instances this is most often seen as the tank noticing that someone takes aggro, stopping what he is doing, slowly turning towards the mob (with all the other mobs hitting him in the back) and then standing like that for a couple of seconds while probably trying to target the stray mob, for all I can guess. This is exactly what a lowbie tank should do, try to handle stray mobs, and even if they don't do it pretty, they're at least trying which really is the only way they're going to learn how to do it at all. It does become a problem when people react this way in higher content however, fortunately that does happen rarely.
4. I stood in fire?
Just as multi-tasking is something you have to be good at if you intend to tank, situational awareness is possibly even more important. Panicking because you don't know what to do when everyone around you has aggro except you is one thing, not even noticing that everyone around you has aggro is even worse. In low levels I can only assume that this is because of a mix of lack of experience and lack of decent tanking addons. Some sort of addon that helps you distinguish between targets that have aggro on you and that have aggro on someone else is basically a must have when you're tanking. But situational awareness doesn't limit itself to noticing when your healer is dying, but when you yourself is dying too. Noticing that you're standing in a big pile of fire is just as important for you as for everyone else in the group, although it might be the toughest for the tank to notice because the boss usually covers the entire screen (I can only hope Blizzard soon decides to make bosses semi-transparent as in other games). As the tank you have to know what happens at the far edges of your screen. Unlike the healer who can comfortably tunnel in on their healing frames, the tank really must keep an awareness to everything that happens, while at the same time try to keep addons around to help out with that awareness. It's a fine balance that I find most tanks struggle with their entire tanking career.
3. The One Man Army
This is a classical case of "didn't grow out of being level 20" problem. Up until level 60 or so maybe even 70, the tank really usually is a one man army, especially if you happen to be a paladin or dk with decent self-healabilities. You'll be the one who deals the most damage (by far at low levels), sets the pace of the instance and overall runs around like a maniac killing stuff. Nothing wrong with that, I do that myself and I can only imagine that that actually is a big part to why I enjoy re-leveling prot warriors over and over. The problem is of course when tanks still think they don't need the rest of the group after level 80. There will always be exceptions, but most tanks actually do need to care about the healers mana, some cc and where the rest of the group is before pulling. It's also not ok to assume that just because you had a vastly overpowered group the last time you did this, it'll be ok to treat the next group the same way. I can't say that a certain class of tanks suffer from this behavior more than others, it might be dks at early level 70s then, because they're usually really strong all through their 60s and don't actually have to have tankgear or tankspec to be able to tank. Wrath is getting really easy now too, but all of these tanks will get a surprise once they get to Cata instances.
2. Hurtful tanking behavior
I would've put "using Exorcism while tanking" here, but (un)fortunately it's paladin specific. But all tanks can do things that are very bad for their tanking, or at least for their survivability. The one I see most often is simply turning your back against the mobs. Again, this is something that many tanks "learn" from lowbie tanking. On low levels, mobs rarely do enough damage for you to care where it stands for any other reason than you being able to hit it. As soon as you wish to move from point A to point B you simply do what everyone else does - turn around and run there, not caring about having ten mobs hitting you in the back. Problem is, I know a couple of tanks that still don't know how to properly move from point A to point B all the way up to raiding! They probably learn damn fast that turning your back towards a raid boss isn't as forgiving as turning your back at some trash, but they've never done anything else and so they don't know what else to do. They start backing up instead. I don't know how many times I've heard the raid leader tell the tank "don't backpaddle, strafe!" and the tank just doesn't know how to do it. I'm definitely not saying it is easy, strafing around with your entire screen covered by someones crotch while trying to avoid shit on the ground. But that's probably exactly the reason why tanks should try and start practicing it as soon as possible, and not while they're standing there with the entire raid group counting on them.
1. Tanking is only about keeping aggro
What seems to be a typical way of thinking for many new tanks is that all that matters when you tank is that you keep aggro on the mobs. What they completely forget is the very reason to why keeping aggro on the mobs is so important in the first place - to minimize damage to the group. By focusing damage on one target, that also has tools to lower their damage taken, the healers will have a lot easier time to keep everyone alive. A while ago however I did Nexus on my druid and noticed that I had way less trouble healing the group when everyone was taking damage, than when the tank was desperately trying to keep everything on him. Because unfortunately, keeping aggro is far from the only thing a tank has to think about. If you take more damage than the healer can handle, then the mission has still failed. Sometimes, although rarely, it is actually better to let some dps offtank mobs than to try to grab everything yourself, this is especially true for lowbie instances. But what is even more important is to make sure you know exactly how to use your damage reduction cooldowns. The cooldowns are not some sort of bonus that are there to look pretty. They're a crucial part of your tanking rotation and you should know them just as well as you know your Mangle/Crusader Strike/Devastate/Icy Touch. At lower levels most tanks don't even have cooldowns. Warriors for example don't get their Shield Wall until level 48, and using it usually means turning the healer from "bored" to "comatose". Unfortunately this means I often meet tanks who either don't think, or just don't know, how important cooldowns are in endgame. Especially when raiding should tanks always keep their hp in the back of their heads and instantly lunge at their damage reduce cooldowns when they drop too fast (especially if you're prone to the issue of nr 4 in this list). I know very few tanks who are real masters of their cooldowns.
(Disclaimer: This was written before the aggro nerf announcement, but imo it only proves my point).