A caster who tries to tell you how to maintain agro.
Gah! Just "gah!". I tend to respond with "<caster name> nuke now! nuke nuke nuke! go go go!", depends if they have a sense of humour or not.
EDIT: Just thought of something worse - casters who get agro and won't stand still. I have actually had to resort to shouting "STAND STILL!" at some in the past - how hard is it to grasp that if you're running around it is harder (and with lag impossible) for me to get into taunt range of the mob and save your life? Yeesh