1) Look at your healer's icon on the party bar (healer is the guy with the + on his icon) - see that strip of blue where his rage should be? That's his mana - if you pull when he has very little of it then odds are you will get yourself and everyone else killed.
2) Staying on the theme of healers, healers also have this limitation to their spells called "range." Range means that if you run too far away from the healer because he's sitting down and drinking to restore the blue bar from item 1 then his spells can't reach you. There's a related issue called LOS or line of sight, meaning that if the healer is doing that sit and drink thing and you run around a corner or into a room then his inability to see you makes him incapable of healing you.
3) Warrior and Pally tanks - sword and board is your friend, yes, the DKs look kewl tanking with a 2h weapon but you're not a DK, you need a shield. If you don't have a shield just pick up the hunter and use him as one - not only does this set you up with a shield but it also prevents him from randomly firing into other groups of mobs and wiping the party with overpulls.
4) Druid tanks - yes, you aren't doing ubah damage in bear form against a single target like, say, a boss and the DPS suck so you're tempted to go kitty form to finish the fight faster as well as show the deeps what real DPS is. While that sounds like a good idea, there's one tiny problem with it - kitties don't have the health, AC and damage mitigation of a bear, meaning that there's a very good chance the boss will kill you as the healer is incapable of keeping up.
5) SK tanks - most of you really don't know what you're doing, please just accept the longer queues and go back to a DPS spec.
6) Gear - resilience doesn't help against mobs and while yes, druid tanks do tend to wear mostly DPS gear, you warriors and pallies can't. That means no PvP armor and no wearing your DPS leveling armor either - your job is to hold aggro and not die, your place on the DPS meter is irrelevant.
Those are the basics - once you've mastered these advanced concepts then you'll only be an idiot tank instead of a moron - please find a "tanking for idiots" thread and see about upgrading yourself again - thank you.
(Yes, I've been having a horrific run of ****** tanks.)