I've been looking for a profoundly simple and elegant synopsis of what we colloquially know as Web 2.0 and I found it in an interesting place. The following is from the Web Standards Group email list available at
http://webstandardsgroup.org:
Web 2.0 is basically what Web 1.0 was meant to be, before it was hijacked by commerce, i.e. collaborative information sharing, social networking, etc.
The difference is that nowadays we have Web technologies which make this much easier and more extensive, e.g. Wikipedia and other wikis, blogs, rss feeds etc.
I guess, from a design point of view, Web 2.0 is creating Websites which incorporate these technologies.
Stuart Foulstone
And to me, that sums it up nicely.