Did a bunch of shit. In Wrath, I would clip my way on top of Ironforge's auction house as a horde and spam ranged spells on people to kill them. If I was getting close to dying, I would just back up so they couldn't target me and regain health. A GM contacted me and told me to get off of there after about two hours of doing it, there was a giant crowd trying to kill me. It was great. I once got a 5 player group together and used cleverly timed jumps and levitations to get inside of the Stormwind bank and camp their bankers so they couldn't use them.
In Cata, I remember mind controlling a Naga and kiting him over to the redesigned Troll starting area; he had an ability that you could use to attack members of your own faction. Corpse camped newbies on my own faction for hours.

I also remember an item that spawned an untextured pet that made the most annoying sound as he ran, and you could summon him an unlimited amount of times. Ran around the Auction House with a million of them running after me, omg the butthurt from it was amazing. Also like Smorgeybork mentioned, summoning a whole raid of unwitting low levels right into Stormwind was fun as shit.