Dion Hinchcliffe has an excellent article on The Habits of Highly Effective Web 2.0 sites
Seven Essentials of Leveraging Web 2.0
* Ease of Use is the most important feature of any Web site, Web application, or program.
* Open up your data as much possible. There is no future in hoarding data, only controlling it.
* Aggressively add feedback loops to everything. Pull out the loops that don’t seem to matter and emphasize the ones that give results.
* Continuous release cycles. The bigger the release, the more unwieldy it becomes (more dependencies, more planning, more disruption.) Organic growth is the most powerful, adaptive, and resilient.
* Make your users part of your software. They are your most valuable source of content, feedback, and passion. Start understanding social architecture. Give up non-essential control. Or your users will likely go elsewhere.
* Turn your applications into platforms. An application usually has a single predetermined use while a platform is designed to be the foundation of something much bigger. Instead of getting a single type of use from your software and data, you might get hundreds or even thousands of additional uses.
* Don’t create social communities just to have them. They aren’t a checklist item. But do empower inspired users to create them.