Inflexibility, that's exactly the thing.
There's always that tank who throws a fit and refuses to tank properly when someone pulls ahead of them, or takes aggro off them, or casts Regen on him pre-pull.
Then there's them healers who refuse to heal people who take aggro off the tank, or aggros, or stands in bad, or is too far.
Then there's the DPS not being considerate of the tank (out threat tank initially) or healer (not using pots, not using defensive abilities, not doing anything helpful) and proceeds to do dumb things (mah DPS!!!) and remove themselves from the fight.
edit: and for any hybrid, the player who refuse to think their role is for anything but their own role. Looking at you, cleric stance haters