Excellent Joel Spolsky article about the design decisions to build the Fog Creek tech support forum.
http://www.joelonsoftware.com/articles/BuildingCommunitieswithSo.html
He talks about small touches designed to improve the conversation and the social space:
* a simple design to eliminate impediments to posting. "That's why there's no registration and there are literally no features, so there's nothing to learn."
* no feature to "email replies to post" -- that kills discussion, since users never check back
* no branching, since branching makes discussions confusing
* topics listed based on time of original post-- drives a life cycle for conversations
* need to scroll through all the posts before you respond --means users are more likely to respond in context
* no "preview mode" -- means users are more likely to be careful before posting
* "reply" doesn't quote previous post -- reduces redundancy
* human moderation -- need to remove troublemakers, much less socially and technical complicated than Slashdot