I've written before about how easy it is to become addicted to that leveling-up ding, and suggested that it's the main reason we play MMOs. But the Penny Arcade guys wrote on Monday about two types of gamers -- those who play to beat the challenges, and those who play to see new content. Both of these are sort of against the ding mindset, with its reward of repetition. I've realized that I'm a new content sort of person, myself. That's probably why I get bored around mid-level so easily; the tasks are longer, and usually more arbitrary in nature. Instead of killing 10 mugwumps, I get to kill 50 -- meh.
So what's your reason for playing? Is it reducible to any of these reasons? What's your anti-ding?