More than all other classes a tank playing bad his role will surely lead the whole group to a painfull death
and if you put in that the tank usually also has the duty to pull mobs you can really see how a bad moove could cause serious danger to your grp
Take a look to this short guide
<-- Link --> for some basic infos
btw if you stick to few guidelines rules you'll do fine
1) go in the interface menu and enable the showing of threat percentage and enable floating text
2) when you're in a team with other classes that could cover this role always talk with others to choose who's tanking
3) choose who's pulling mobs and stick to that decision... if you need a bit of time to make your decisions ask others to just wait you to choose the correct way to pull or just leave this duty to someone else... if everyone starts to pull you'll soon find your team surrounded by mobs
4) ask others to don't intercept the mobs you're pulling b4 u have them in the range of your melee weapon... or they'll take aggro away from you and you'll need to run after the mobs and spend precious rage points trying to take back the mob's attention
5) ask others to always try to attack the same mob your attacking... this way they won't take attentions of other mobs and you will less likely need to run around trying to take aggro back [the assist command comes very handy in this cases... ask the others to select u as target, press "f" and don't deselect u... this way they'll automatically target the mob your targeting]
6) always look at the priest [or healers in general] icon... if it's glowing red means he's being attacked... leave everything you're doing and go to take the mobs away from him... same rule should be applied to all casters but healers always comes first
7) a tank is supposed to lead the grp in the dungeon... make macros for things like "wait", "follow me", "charge" and so on... so u'll just need to click an icon on an actionbar without the needs to type it down... those few seconds could save your grp to fall in an ambush or leave you behind
that's it... at least that's wat i learned till now
let me know how that works... and be sure to share your tips