Here's an idea. Stack Overflow should have an aggregated stream of updates concerning posts an user is "involved with," as well as other stuff (getting badges, etc.).
Currently, users only get notifications when their own questions are answered (unless I've missed something). But if I answer or comment on an answer/question, I'm usually interested when someone else comments on/answers that question.
Having a news feed page where all this is aggregated into a nice river of news, similar to Facebook's, would be really great, I think, and would increase activity on questions and comments overall.
Sure, it might get a bit hectic for popular questions that generate a lot of comments and answers, but you could have logic for managing that. For example, not showing additional answers after n posts, or after the question is a certain number of days old, etc.
You could mark people as "friends" to have them show up in the feed. It would be nice to see what your developer friends are answering and commenting on in your feed as well.
This might sound a bit too much like Facebook for some, but it's just to be able to easier follow and participate in discussions and answers. Everything that gets people more involved is good in my opinion.