The main difference as a tank between running instances and tanking in raids is that in raids you get to work with more people. In a raid, you are part of a tanking team. You will very likely have a private tanking chat channel, which will be used for assignments as well as small talk.
You will probably be given an assignment on every trash pull as well as every boss fight. But there are some general rules of etiquette when working with other tanks — people won’t necessarily tell you these in advance, but you’ll quickly find out when you have crossed the line.
1. Never taunt a mob off another tank (unless by prior arrangement), it’s rude. It is the tanking equivalent of telling someone they’re doing a bad job and you could do better. If you taunt something off another tank by mistake, apologise after the pull. We all do it, but it’s polite to say sorry.
Note: this is a little different in Warhammer because taunting makes you do more damage. In that game it’s more common to have taunting rotations (or just assume everyone will taunt when their taunt comes up).
2. In a trash pull, tank the mobs close together unless you know in advance that one has to be taken to a safer distance. This makes it easier for the dps to use AE, easier for them to move from one mob to the next, and also easier for other tanks to use AE parts of their rotations (why would you want this? It’s extra damage/ debuffs as much as anything else). If you insist on taking your mobs off to their own special corner without prior arrangement, expect to get some snark.
3. If you’re in a raid and you know another tank hasn’t tanked a particular mob before, make sure they know any special tanking tricks that they need to know. The raid leader may not be a tank and may not think to remind them.
4. Tanks are a TEAM. You have to work together. You stand as a team, you fall as a team. It’s not a competitive sport like dps (or healing, although that really shouldn’t be competitive either). You will tend to find that most tanks are pretty supportive of other tanks — there are a LOT of helpful blogs, websites, bbposts, gear guides, and so on. Yes there’s a lot of player support for most sides of the game, and raids are teams too, but tanks stand together. Don’t refuse a request for advice from a less experienced or lower level tank.
5. If you see another tank in trouble or low on health, be ready to step in if needed. This can go against rule 1, but it’s OK if you saved a wipe.
Tanking is like sex, it’s more fun with partners :) Happy tanking!