When I was at the university, I studied Amy Jo Kim’s points of view about communities and social psychology.

Today Amy Jo is co-founder of Shufflebrain, a company which builds “smart games for social networks”.
She gave this keynote about “putting the fun in functional”, which I think is a pretty good basis for playful design. Enjoy the slideshow, and note the sentence at the end: expect to see more “serious” applications that feel like games… …and games that incorporate communities.