The best article I've read on the subject is in the link below.  In summary, the article cites the following 7 factors common to Web 2.0:\n\n* Services, not packaged software, with cost-effective scalability\n* Control over unique, hard-to-recreate data sources that get richer as more people use them\n* Trusting users as co-developers\n* Harnessing collective intelligence\n* Leveraging the long tail through customer self-service\n* Software above the level of a single device\n* Lightweight user interfaces, development models, AND business models
What's your definition of Web 2.0?