I think this would be a great idea...
WordPress.Com could hold a weekly/monthly contest for a subscription. Each contest would have a different subject.. maybe the first one being that you had to post an essay about why WordPress.Com is so cool, and submit it. We could elect a small judge team to review the entries, and then there could be 3 top prizes:
1st place: 10GB Upgrade
2nd place: 5GB Upgrade
3rd place: Custom CSS
So basically, every contest would require you to write some sort of post and submit it. It would be reviewed by judges, and the top posts would win the above prizes.
Who would be interested in participating in something like this?