Rowan has a great article up that digs a bit into the difference between intrinsic and extrinsic motivations for play. Penny Arcade fortuitously has a similar article from a game developer (for the game Don’t Starve, a curious game that I’m looking forward to playing).
I come down firmly on the intrinsic side. I love Minecraft because it lets me just go do stuff (especially in Creative mode where I can fly and have access to everything). I love Burnout Paradise because I can just go drive around and see what the city holds. I love SSX 3 because you can start at the top of the mountain and just snowboard down to the base, purely exploring the terrain. I love flying in World of Warcraft. I love just moving around in game spaces.
I want to do things that I do because I enjoy doing them, not because there’s a reward for doing them. Living and doing things you love are rewards in themselves.
Carpe Diem, as it were. Live, don’t just survive. Play.
…though I can admit this is fun. Metagaming the achievements, whee!