I like to explore worlds. Up until recently, the only games that ever let me explore vast spaces of the unknown were RPG's (started back in the day with Ultima, Pool of Radiance, Questron, Wasteland etc..). Nowadays there are even shooters (Boiling Point although some would probably call it an RPG also, Far Cry, Crysis), action adventures (the GTA series mainly) and racing games (Test Drive Unlimited) that let me do these things.
Combine the exploration with a gripping story, memorable characters and a character development system that sits in the background and shuts up (no D&D for me, that is!) and you've got the ingredients to make me drool. Oh, and boobs.