1. Have at least one tag cloud
2. Allow users to add their own content without any form of editorial review.
3. Make sure users can add tags to content
4. Allow users to rate content and add comments.
5. Be more than just another blogging tool or social bookmark tool
6. Support friends and groups
7. Allow users to create lists
8. Use AJAX for something, especially for text field fill ins
9. Make sure users can upload a profile picture and fill out some questions about themselves
10. More important – make sure every user has a My page.
11. Have RSS feeds for every possible way to dice up your content
12. Integrate at least one Web2.0 API listed at ProgrammableWeb such as Google Maps, Flickr, or YouTube.
13. Add Forums and minimize integration with the rest of the site
14. Implement Google AdWords
15. Develop in php, Ruby on Rails, PERL, or Java/Tomcat

And my Favorite –

Just claim that you’re a Web 2.0 site!

I’m sure I’m leaving out a bunch of stuff…

In all seriousness, O’Reilly’s What is Web 2.0 and the entry on WikiPedia’s Web 2.0 is a good place to start if you want to learn the real essence of Web 2.0. Be ware. Web 3.0 is coming.