It seems that no matter what game you play, no matter how selective you are in who you play that game with, you eventually run into a common problem: one or more of the players starts to act like a drama queen. They have nothing but criticism about everything that everyone else says or does, they never make mistakes (and immediately launch debates about how it is always someone else's fault), and nobody is more deserving of praise, glory, or reward than they are.
The sad part is that most of these prima donnas don't even understand the damage they cause to their own enjoyment of the game in acting this way. Let's set aside the old tripe about "it's only a game" for a moment. Yes, it is a game, but like any other team-based game or sport, if you don't intend to at least try and play it to the best of your ability, you honestly shouldn't bother in the first place. So I can understand when someone is dragging their feet in doing something vital, and people get upset. It's no different when you are playing tabletop D&D and someone just plain isn't paying attention to what is going on around them: it is annoying to everyone, slows things down for no reason, and makes the game boring for everyone involved.
Having said all that, drama queens in group/raid content are the opposite extreme. Let's examine some of the worst offenses.
Telling someone how to play their class/role: This is probably the most common and usually the most offensive thing people do to other people. It isn't that it doesn't need to be done (often it does, if someone got to end-game without taking time to learn what they can and can't do), but drama queens don't bother to do it properly. They take a very mean and offensive tone of voice about it, and don't realize that all this does is make people less likely to be responsive to them in the future.
Never taking responsibility for anything: Less common (but, to me, more annoying) is the person who is never at fault for anything that goes wrong with a bad pull or whatnot. It seems to happen more when there is a lack of communication between members. One example I've seen is that people will get defensive about using AOE when someone uses CC on mobs. In such cases, it is everyone's fault for not communicating better ahead of time to know what tactics or strategy will be used.
Being greedy: The worst offender of the lot are those people who seem to think they are entitled to things in the game. This is also the most ridiculous claim, since it has no basis in fact or common sense. The only thing your $15 bought you was a chance to play the game with other people. Everything else is arbitrary. Even worse, some players will act down-right retarded about virtual loot that only gets outdated later on, when new content arrives.
It is easier to deal with these virtual divas by establishing some ground rules in the group/raid that will help deal with several issues at once:
1. When you first form up the group/raid, encourage communication. Ask folks what their spec is, professions (which may affect what mats they are looking for during the run), and other useful info. This not only helps everyone understand what they are supposed to do and what they may need/want, but you can get a feel for how people talk in chat (or voice, if you have that option) so that you can talk to them easier.
2. Establish loot rules as early as possible. Make sure that everyone knows what proceedure is. Just because you think it is dumb to pass on BOP gear and roll seperately for it doesn't mean that others feel like you do. Make sure everyone is clear on how things will be done with loot. This also makes sure that everyone gets a chance to speak up if any found loot is an upgrade to them.
3. If something goes wrong, don't jump down anyone's throat. Start by telling them what you yourself did wrong. For example, even if someone else broke CC and caused adds to pull, you may tell them "Hey, sorry I couldn't DPS fast enough to bring them down." This one thing seems to work wonders for the attitudes of other players; I've seen really poor attempts on a boss improve dramatically after several players fess up about even the smallest shortcoming they had.
4. When someone starts to act like a diva, don't let them go unchecked. If someone acts overly rude, speak out against it. If someone continually rolls Need on things that need to be Greeded or passed on, a firm reminder not to do so needs to be given the first time it happens. No need to be rude in standing up against such things. After all, anyone can have an "off" day or make a mistake clicking on something they didn't mean to. Just make sure they know that such things aren't for the good of the party.
5. If one or more players won't correct bad behavior, just quit gaming with them. Leave the group/raid. You may get bad-mouthed at first, but eventually anyone who has any common sense will see who was in the right, and who was in the wrong. Besides, if you leave, you may encourage others to leave, too... leaving the bastard stranded with nobody to help them.
The best bet when dealing with a diva in any game is to just take the high road. Remain as polite, patient, and fair as you possibly can. This helps other players to understand that nobody has to put up with crap behavior in a game where everyone is supposed to be having fun. And more often than not, others will remember you when they need a group themselves.