Omnipotence, omniscience, and having to deal with users' complaints day in, day out; being a GM has its ups and downs. If you suddenly found yourself in the shoes of a GM, for one single day, what would you do with your power? Is it time for revenge, or would you seek out those in need of help? Would you run rampant on the forums? Glide invisibly around the world?
Of course, GMs can't make gameplay changes, but you could perhaps escalate a particular pet proposal that's in the pipeline. The notion of character transfers is one I'd like to put some weight behind, plus some mechanism for international co-operative play. With a GM's power, I'd love to just explore the world, invisible and untouchable. A day might not be long enough, though.