A couple years ago, a friend of mine created a simple webapp to host social workout challenges. We would record the number of push-ups, squats, etc. that you did each day, and the app would give you points to compete with your friends.
The app was fun for everyone the first couple weeks, but there was one recurring problem: the most fit users dominated the rankings every week. Ultimately, for those trying to just remain active, it became demotivating to end up in the same rank every week. Not only was losing discouraging, but the app also gave no incentive to grow - the priority was points, however haphazardly you could obtain them. A few months after it started, the challenges died down and many of us were still struggling with maintaining fitness.
During my UX class, when choosing a first design project, I thought of this app again and I wondered if the product could be redesigned to use competition better.