I'm listening to a presentation Joshua Porter gave at an event called Buy, Hack, or Build: Optimizing your Systems for Your Users and Your Sanity:

"...I wasn’t going to define Web 2.0, as most of the definitions I’ve heard are ambiguous, to say the least. Instead, I focused on four aspects of Web 2.0 that I think are most relevant."

Web 2.0

Well worth a listen: Web 2.0 for the Rest of Us (.mp3).