I think of the good ones as those that make me happy, and the bad ones as those that I do even though they're not making me happy. Sometimes that's the rat-pushing-button syndrome, but sometimes I'm obsessed with something even though it's never been very rewarding. EverQuest comes to mind as an illustration of the latter. It wasn't a bad game, but it wasn't ever so much fun that it made sense to let it take over my life.