Boxes and Arrows has a great article on the origins of the term Web 2.0 and how it defines a medium and culture in modern technology.

But what attracts users to donate their time and energy to contribute to Web services like Wikipedia or Flickr while not doing so to corporate services? Psychology and economics teach us that there is no such thing as altruism – whatever people do will create a personal return of value for them. This personal value is measured by individual criteria. Respect and prestige, personal reputation, political beliefs or desires, and of course monetary incentives influence the decision as to whether their contribution creates this value. People create an article in Wikipedia because they believe that the topic is interesting or important or because they want to see their name in print, and put pictures on Flickr because they want to share them with others, thereby influencing how they are perceived by others.

http://www.boxesandarrows.com/view/the-trouble-with-web