Hopefully we all know that body language says a lot in real life and have some ability to read and control it. But what about in WoW?
I dislike other characters standing on me. It's so intrusive. Give me some space! Too many fights require some or all of the raid to cluster into a 5x5 square.
I face people while trading. Facing away just seems rude.
There are more practical forms though.
Misdirect or tricks of the trade are how a DPS can say "I'm ready to go, please go soon" without the standard, rude "gogogo". It's more subtle and implies that they are offering to help make the pull work better for you, rather than merely rushing you.
Conversely, a rogue wandering among packs of mobs, sapping randomly, is either pickpocketing (and the fact that he has the time for this means you're quite slow) or he's trying to purposefully express boredom. A constantly growling pet racing back and forth may be due to a hunter sending out and calling back a pet in rapid succession, as if he can barely contain his eagerness/impatience. Casters who cast and stop frequently are doing something similar. It sends the message "I want to go, let's go, why are we not going, GO!"
Either misidrect or tricks of the trade on a healer means someone is either making a joke or is a total douchebag who should be kicked immediately. Use contextual clues to differentiate; doing this while the tank or healer is afk indicates douchebag while if they are in the same guild as the healer it indicates a joke on vent that you didn't hear.
Prayer of Mending is how a priest says "I'm ready". It sounds like a bell to me, which doesn't matter in this context, I just wanted to point it out. Power word: shield also works, especially if it's the sprint bubble from a holy paladin. BTW, I love the sprint bubble. I use it to catch up when people pull while I'm drinking. I want that last 10% mana, dammit!
Vigilance is a warrior's way of saying either "Your DPS is clearly the best" or "You are terrible with aggro management but I feel accommodating". Either way, nice guy.
A tank standing still is ambiguous without context. If a DPS is ahead of him, he's saying "You ran ahead, go for it". This is an especially cold-hearted expression if the DPS have pulled a mob and are now engaged in combat. If the tank is ahead of everyone and has run close to aggro range and stopped, he is likely trying to figure out a pull. This is especially likely if he seems to be moving around a lot and switching targets. If he instead is standing still after the previous pull, especially if he isn't targeting anything, he's probably waiting on a cooldown such as death and decay or divine plea. Or he's trying to say "DPS, stop saying 'go go go', it's really annoying".
Some would just call all this being passive aggressive, but to them I say *look of disgust*. So there!