What creates a community site? I think it's extremely simple: community sites are aggregations of people who are interested in what each other has to say. Actually achieving a community site is anything but simple. As I said in a proposal for an upcoming conference:

We are using simple behavioral approaches in combination with technology to bring coherence to the remixing process. In class, we come to consensus on key tags. Participants also consult the class tag cloud and use the same tags as the items their contributions follow. Thus, we have an evolving and self-organizing archive of class attention.

In other words, it's more than just aggregation. It's getting people to contribute to the aggregation, pay attention to it, and respond to it. In a classroom setting, these seem easy because most people think you can just assign part of the grade to them. There's some truth in that, but that really only starts things. What makes them keep on going is the idea that people are paying attention on the other end.