Here's a quote:
Keeping the Rewards Coming
In the absence of gaining levels, there are still ways to reward players who want to continue questing or playing through quest-like experiences. Here are some things (there may be more) that might encourage players like this to stick around:
- Participation in an epic story
- A sense of progress
- Discovery of something new or unexpected each day
- Earning character customization (including cool mounts!)
- Earning fun toys
- Making your character more powerful
He doesn't even mention anything that would be MMO specific. The entire list could easily be applied to a single player game.
Oh - and he seems to think that it is all about rewards, rewards and rewards. It seems rewards are the only thing a Blizzard employee can come up with he tries to understand why somebody plays a game.