Sometimes I wonder if it is the only aspect of the game that some people know how to do. The examples sure are more numerous than one could list, e.g. horde vs alliance. Aye, seems rediculous at first but people tend to take it very seriously and the hatred for anyone not of your faction is thick enough to cut with a butterknife. Same goes between guilds, especially if you are unlucky to be in one that aroused the ire of the community, you might not even be accepted to PuGs because of it.
One of my all time favorites is asking or just talking on trade or the general channel. The trigger can be almost anything really, like asking if someone wants to take part in your group quest or just asking directions where a specific quest is completed. Perfectly understandable isn't it? After all, though it reads "Suggested for groups " doesn't really mean it is meant for groups. It is perfectly soloable in the uber gear that we are all privied to.
Other good examples include pvp, which is an instant combustion for the losing team, certain classes, of death knights are a prime example (after all, was our fault that we were so overpowered back in the day when earth was still flat), poor gear, mistakes on instances and raids, or just happening upon the same bond that someone else is already fishing in.
I understand many play the game with great passion. Personally, I am very much the same. I love playing it and I try to do so every day, but it does not make me hate the world and certainly cannot find it in myself to degrade and ruin someone else's gaming experience just because I am prone to psychotic episodes. In my mind, people like this have no place in the game and personally hope Blizzard would implement more strict tolerance for it.
It is still a game, one have volunteered to subscribe to, and the joy of playing is the only valid reason to continue playing.