The Rules
Jeff Jarvis nails two fundamental concepts of Web 2.0:
Jarvis' First Law: Give the people control of media, they will use it.
The corollary: Don't give the people control of media, and you will lose.
Jarvis' Second Law: Lower cost of production and distribution in media inevitably leads to nichefication.
The corollary: Lower the cost of media enough, and there will be an unlimited supply of people making it.