Micro Persuasion points out an interesting defininition from information week for web 2.0.
"Web 2.0 is all the Web sites out there that get their value from the actions of users."
Here's my definition of Web 2.0 - a web site that makes money - because before almost no web sites were and they were still seen as being legitimate.
But seriously, web 2.0 is all about getting more interaction from users. Getting them more addicted to this socially isolating pretend world of information.