1 - Please don't pyroblast the mob before I've even touched it.
This isn't just limited to pyroblast. Also don't frostbolt it, don't nuke it or DoT it up or stun it before it gets to me and start stabbing wildly before I've had a chance to so much as throw one single, solitary sunder on it. I don't want to see a big burning horned skull appear over a target's head just as I pull it. Especially not when I'm pulling it with a gun or a thrown weapon because, I guarantee this, even your weakest DoT will pull the mob off of me when all I've done it annoy it with a throwing knife. I don't know why even smart players seem to have a hard time with relapsing and doing these things when they're not being yelled at by a raid leader, but just, for the love of all you hold dear, stop doing this. It will not kill you to wait five seconds for your tank to make physical contact with the mob, really, I promise you. It will not even slow down the run. Since the tank won't be forced to chase an errant mob and blow a taunt to get it back, he'll be able to establish a better rhythm of specials and will have some rage for when the next mob is loose. Furthermore, if we're doing a multiple mob pull and the first target is yanked away early the warrior won't have the rage he needs to lock down the other mobs. There's no reason to be impatient when it will just lead to mass chaos later.
2 - Please don't randomly DPS mobs for no reason.
I didn't put all those colorful symbols up over the mobs because I expect a leprechaun to provide me with a delightful balanced breakfast, I did it because we're going to kill them in a specific order. Yes, I will be tanking two of them. No, that doesn't mean you should try and alternate DPS between the two of them. Kill the one with the skull first, he'll be the one I'm focusing most of my effort on holding. I know you can pop evasion or ice block if you get aggro, that's not the point. Unfortunately for me, I can't just let mobs chase you around, because if they kill you or lose aggro on you they might go eat the healer, especially if they've been run into the proximity of that aforementioned healer who has been spamming heals on you. I don't want the healer to die. The healer doesn't want the healer to die. So please, please, stop hitting every mob. You can make an exception for DoT's on mobs after they have been aggroed by the tank. If I'm standing in the middle of three mobs, tab sundering and using thunderclap, then it's okay to put DoT's on each mob to soften them up. It's not okay to start burning them all down at once.
3 - If you have a tanking pet or minion, please learn when to use it and when not to.
I am not one of those warriors who freaks out about pets in instances. There are lots of times when you want the tanking pet to tank things. Is there an extra mob we'd like to tie up for a few seconds and we're out of other CC? The pet can tank it and we'll just make it second on the kill priority. Is a mob breaking loose and about to kill the healer? By all means, fire up growl and take him out. Otherwise, unless your pet is doing one of these two things, please turn off whatever form of taunt it may have. If a pet is constantly growling and turning my target, I have to burn taunt to get it back, and that's a few seconds of cooldown. It may not seem like much, but if I have to fight your Felguard for aggro and then another mob runs to devour a healer, we'll all wish I had my taunt up instead.
4 - Know when to use stuns, shields, and other abilities that deny me rage.
If a mob I am tanking is stunned when it is too far away from me, I have to move to get back to it, which makes multi-mob tanking even more complicated. It's generally best to wait to use stuns or interrupts for at least the first ten seconds of a pull unless they're being used as CC on a mob that's not being tanked, because a mob that isn't hitting a warrior isn't helping him generate any rage, which means he has less rage to use to build aggro. Once a warrior has had a chance to sunder every mob he's expected to hold aggro on at least once, it's okay to stun the mob to keep its damage low. Similarly, when using PW:S, be careful to not use it directly before or just after a pull. I know it's not considered to block a lot of damage, but in those first few seconds unless it's against a boss that can actually one-shot the tank, it's not worth using it. Feel free to throw it up after aggro is fairly secure, however.
5 - Please do not taunt my mob.
This goes for warriors and anyone else who has a taunt. Do not use it on my mob. See the post above for pets. Use it to offtank, use it to pick up a runner on a healer. Don't taunt a mob I have solid aggro on. The healer doesn't know you've decided to be main tank all of a sudden, she or he won't know to heal you, you'll take a lot of damage and maybe even die, and then the mob may well go for the healer who desperately tried to keep you alive. This is the worst case scenario, yes, but the best case scenario is that I just taunt it back: why waste everyone's time?
I'll wrap this up with advice to my fellow warriors who may still be learning the tricks of tanking.
Don't rush the party.
I know your rage bar is declining. Suck it up. They need mana to function. If they can't function, you can't kill anything. You'll get more rage once stuff starts banging you in the head with blunt objects or lighting you on fire, it's a renewable resource.
Don't set up CC and then charge in and thunderclap in the middle of it.
We didn't sheep and sap those guys because we're barnyard enthusiasts.
Pay attention to the whole battle, not just the part happening directly in front of you.
Tunnel vision is a big problem for warriors as they start learning how to tank. It's great that you kept those two mobs agged on you the entire time, but if you let a third mob eat the healer or kill the DPS, you've failed. You're not there to glue mobs to yourself, after all, you could wander around any zone you like solo and have as many mobs attacking you as you could possibly want, all the colors of the rainbow. What you are there for is to keep the mobs from killing the rest of your party while your party kills them. You're better off letting a rogue dodge tank a mob for a few seconds and running over to pull another off the priest than you are standing static in one spot. The fight is fluid and you need to adapt to it.
Seriously, don't rush the party.
Man, they hate that. Sometimes they'll let you run ahead and die while they finish their drinks.
Bring consumables and use them.
Bring buff food, buff pots, heal pots and bandages. Be willing and able to use them. Consumables are one of the few advantages a warrior tank has over a bear, if you're not out there farming them up or getting them some other way, you're a less effective tank. Level your cooking, level your first aid, get an alchemist alt if you have to but keep stocked as best you can.
Okay, that should be enough for now. We could go on and on about being willing to stance dance and sword and board it, but I'll save that for another column. Feel free to comment on anything I may have missed... I know everyone makes mistakes, and I can't possibly predict them all.