As far as I remember web 2.0 was all about user generated content spruced up with Ajax experiences:

Blogs (chronological as opposed to more freeform web sites) were arguably the first.

The comments, follows ("blog rolls"), tagging, ratings and third party sites providing the same like del.ico.us and digg.

RSS also was a web 2.0 thing in my mind at least.

I think most people classify Facebook as web 2.0 as well but in my mind they aren't as much web as a silo built on web technology.