Web 2.0 = When the back button doesn't work

At his ZDNet Testbed blog, David is soliciting input for the top ten differences between Web 1.0 and Web 2.0. He also posits his own definition:

...when people ask me what Web 2.0 is, a lot of times, I say "It's when the back button doesn't work! (something that usually works with Web 1.0… but not always). The other thing I say is that I don't think there's a trade or service mark on "Web 1.0."   

By that meager defintion, an AJAX page would define Web 2.0. I am happy to go along with Tim O'Reilly's latest crack at defining the concept, but let's just get on with the innovating the Web as opposed to lexicographical exercises. That said, it will interesting to see if anyone can come up with a better way of capturing how the Web is transforming before our eyes or pointing out where the gaps to making the next leap exist.