I think I got it. I am sure I am late to this "hot" topic ;)
I have been trying to define "Web 2.0" in my own words. Until I do that, I don't feel comfortable even talking about it. People talk about it but rarely, it seems, understand what it stands for, and that included me until now (hopefully).
First of all, Web 2.0 is "a system that let users build a information mass which creates greater gravity to attract more information (via visiting users)"
The essense of gravity is "fundamental forces by which all objects with mass attract each other" as wiki articile says.
Unlike the physical gravity, in web world it is informational gravity: information (actually persons who posseses information) are attracted to a mass of information.
In addtion, Web 2.0 is "a system where information mass grows using both user-contributed assets, user-judgements, and user-activity events".
User-contributed assets are actualy not new to Web 2.0: it is any data user provides explicitly including things like YouTube video, discussion board statements.
User-judgements are indications of user's opinions, typically in the form of positive or negative feedback against data/information available. It includes "thums up" and "stars" you give to things like blogs.
User-activity events are completely implicit: if user adds a URL to a bookmark service, its action generates a statement, that becomes information, being broadcasted to other users. The fact that you watch a video in YouTube implicitly contributes to the build up of information - view count is a automatic voting mechanism.
Web 2.0 efforts should contain some, if not all, of these aspects.