I am almost exclusively a player. I love rules mechanics, and game theory, but whenever I step behind the DM screen, it just feels intimidating -- I'm not the kind of guy who always thrives when put on the spot, and I feel horribly uncomfortable winging things.
Currently, I'm in heaven, though. Playing a fun character in the first campaign of a homebrew world that our DM's been kicking around the rough sketches of in his head for a long while now. I showed an interest in a few regions of the world (race, region, and one of his loosely defined prestige class organizations), and when I started prodding for some character background fodder, found out that the backgrounds for the stuff I jumped at were only vaguely outlined, and fairly open-ended. So I've been delegated to help out in filling them in, which gives me an outlet for my creative juices without having to actually run the game.