I'm sick and tired of people trying to define what "web 2.0" really is, saying it doesn't exist, or it does exist. It's simply a buzzword, and nothing more.

In my experience, here's what the word seems to be:

When the word "web 2.0" is used in a design sense, the person is usually referring to a simplistic and functional look.

When it's used in programming, it's mainly referring to Ajax or RoR.

When it's used in another sense, it's referring to the power of users controlling the site, as seen in social networking sites, or web apps.

But it's just a word that I wouldn't worry about.