One of my favorite WoW blogs, HoTs and DoTs, has a great post up about Dungeon Groups 101 -- the very basics of running instances. You may think that there's nothing more basic to the game than getting in an instance with four people and taking down a few bosses and trash, but you'd be surprised. Even in a social game like this, one of the first hurdles newbies have to deal with is joining a group to play together. They worry that they'll do things wrong and that other people will make fun of what they're wearing or playing, and that worry keeps them from enjoying my absolute favorite part of the game.
Cassandri's writeup is an excellent read for anyone who feels that way (and feel free to pass on this post to any friends or relatives you know who've been too leery to join an instance yet). She does do some basic knowledge stuff in there, just hints on the classes and what they can all do -- and our WoW Rookie posts will help out with that stuff too -- but more importantly, she says what lots of new players need to hear: that messing up in an instance isn't that big a deal, and that playing together with others (which is the reason why we're all playing an MMO rather than a single player game in the first place) is more than worth getting past any anxiety around joining a group. I've read a lot of comments like the one Cassandri quotes in her post, too, and I'm here to tell you: if you haven't run an instance yet, it's time to stop worrying about what it'll be like and give it a try.