From a DPS point of view, I'm more concerned about tanks that:

1. Run to the next pull with alive mobs (half HP) of the previous pull chasing them, forcing melee classes to go after that mob (losing their situational buffs) and casters to interrupt their spells. If this happens, I usually refrain myself from finishing off those mobs.

2. Think you should AoE because you can AoE, regardless of their skill at getting and maintaining multiple mobs' aggro, healer's skill and overall AoE damage. Nobody wants to have someone telling you how to play, right? I don't tell them how to tank, so I don't give a shit when tanks tell me how to fight mobs and which spells to use.