On April 24th, Joel Spolsky gave a talk at Google on Stack Overflow. The talk was recorded and is available for viewing on YouTube.
The talk highlights how, from the very beginning, we designed Stack Overflow to be a destination page for web search. We explicitly optimize for "drive-by" programmers who type a problem into a search engine; we're not big on social features, and we don't demand that users create an account and hang out on the site. Most of all, it's about the information -- solid answers to useful programming questions!