Every time a player puts down a train set there armour is fully broken and a troll face appears as the cost to repair.
I am Murloc!
Make Heroic and Normal raid gear grey quality, so all those folks with automatic vendoring on would lose their gear everytime they went to a vendor
Make it so that crossing a zone will auto dismount you flying and ground mounts
Place an NPC near every AH where every time somebody tries to enter to buy something, they get death gripped by the NPC.
Give every other class a bubble but after that they deal 0 dmg for about 30 mins except paladins deal 100% increased dmg.
OR: Spread rumors about a super rare item that drops and after a long search you do get it but it does nothing.
Herald of the Titans
I would spawn magic bananas (the ones that trip players) under every player within a 100 yard radius.