How do you qualify a Web 2.0 website?

How do you qualify a Web 2.0 website? What are musts in Web 2.0? I ask because I want to build something for the community and it should definitely fall into the Web 2.0 categorization. Thanks in advance.

Some people argue about what Web 2.0 means, some think that if a site isn't 2.0 that its worthless, and some thing that 2.0 is a fad. So really I think I'll give a definition: Web 2.0 is a phrase that generally means that the site is more focused on community with information generated largely by the community itself. There is no qualifications but there are some aspects which make the site appear Web 2.0 such as

bright contrasting colors
rounded edges
strong typography, often with font sizes larger than other sites
a lot of ajax and slick effects
simple user interface

and then the site usually accomplishes a few things

attempts to be iconic
tries to make a customized user experience
wants to facilitate user interaction as simply as possible

So the question more is what do want to achieve with this site? There are no musts with Web 2.0, in fact the general user probably has no idea what it means.

My suggestion is this when it comes to this Web 2.0 business: Do what your site visitors need, and do not try to conform to some ambiguous standard. If your site needs a forum, include it, but don't include a forum just because you see other sites with a Web 2.0 appearance having a forum.