In "Code Version 2.0" Lessig has a great section on virtual world spaces, specifically comparing Second Life and There. The major distinction he points out is that the architects of SL let members be the owners of anything they create; it's their intellectual property. In contrast, the creators of There follow a corporate model and take ownership of anything created in There with the exception of other businesses (Nike, etc.) with a presence in There. There's creators imagined businesses playing a central role in providing assets that individual users would then simply enjoy. Consequently, while There is still here it's fizzling as Second Life continues to grow.
A small but interesting point of comparison is the manner of accessing each. They both have their respective client VW viewers but There also allows you to access your account, including assets, help forums, and shopping, through the corporate Internet Explorer browser and its activeX technology.
Lastly, I wish Lessig would had discussed OpenSim, which I'm working on setting up now for my school. He discusses Open Source somewhat in the abstract in other parts of the book, but it would add an interesting perspective to the virtual world discussion.