B is for Bad-Players

Don't be one.

Bad players can be classified by a few key things.

1. Swearing, esp repeatedly. Now, it isn't ALWAYS a bad player, but if they are swearing to the group then they have anger issues/skill issues. If they're swearing as part of their standard means of communications eg. whispers/g-chat conversation, it is not as much of an indicator.
2. Standing in the fire. Yeah- they die. You don't want to be that one.
3. Pulling mobs/trash that the tank didn't want pulled yet. Sometimes, if they know the tank, it could be that they know the load the tank can handle- but often its not a good sign. However, if they only pull one pack in a few runs, it could very easily be a tab-target error.
4. Boasting. Those who claim to have skills generally don't.
5. Mis-gearing. Now, I try not to be an Elitist or a Jerk, but if you are wearing tanking gear and trying to DPS or heal, or healing gear and trying to tank- shame on you. Now, at lower levels it is quite possible- but at end game, gear precisely for your role. Gem and enchant for it as well.
6. AFKness. Now, often real-life can interfere and that is fine. However, if they repeatedly go afk- that can be bad.