Defining Web 2.0 to a Layman

Somebody asked me the other day what the term "Web 2.0" meant. This is a topic that has been debated to death on the blogosphere, since there is no standard or official definition. (Think of this post as being my contribution to that old debate).

Some people will say "It's the next generation of the World Wide Web", but that doesn't tell me anything. I mean, how do you distinguish Web 1.0 content currently on the web from Web 2.0 stuff? For instance, I would consider Google Maps as a Web 2.0 application, while Mapquest is a Web 1.0 site. The distinguishing factor to me: the level of interactivity within the user interface that does not require a complete postback.

Wikipedia has an entry discussing Web 2.0 here:

http://en.wikipedia.org/wiki/Web_2.0

After thinking for a while, I simply told this person that Web 2.0 refers to thick-client functionality that is built using thin-client technologies. IOW, it's a Winforms-level of application running within a web browser.

This isn't a perfect definition, but it did get the point across in this case. Are there any better/more concise definitions available?