Anthropology: The Art of Building a Successful Social Site is an good ReadWriteWeb articl about how Joel Spolsky & co built a successful social site for technical Q&A. One distinctive feature of the site the answers are wiki-editable, so they can improve over time.
The thought and care required to create a thriving community is a best practice in general. The specific items, however, vary by community. The sorts of rankings that appeal to competitive male programmers might be the wrong affordance for a more gender-mixed or female-predominant audience. Facebook, which is more mixed, has “like” as its only core rating. LiveJournal has lots of ways to show emotion. The need to design the affordances for community interaction is a constant requirement; the design itself will vary by community.