People ranked high on my list:
1.Shadowknights/Paladins that are purely spec'd for dps and practice increasing their dps religiously ( I don't know why but this really pisses me off

).
2. Paladins that are spec'd for healing (you're a tank, tank shit).
3. Furies that dps instead of healing (and end up at 1% of overall heals on the heal parse or close to *given example*)
4. Wardens that try to dps instead of heal ( /cut yourself now if you're one of those).