Doomguard Roulette in Ironforge's AH or goldshire was probably my favorite. Fear was pretty funny before it got diminishing returns. watching rogues run around helplessly as dots slowly killed them was one of my favorite pass times. Summoning people off cliffs, under water, etc. lets see... Oh! find a lowbie that was struggling a bit, then use Fear on their target to cause it to run into a pack and curse of recklessness when they aggroed. bonus points if you could convince them you were helping.
Warlocks were the best at ruining things for other people, lol.