Fred Wilson outlines what a reputation service needs to do:
1) Identify things you can trust. Its not just about people, but people are an important subset.
2) Identify things you cant trust.
3) Incorporate user feedback.
4) Incorporate other data that is relevant.
5) Allow users to set their own standards. For example my idea of a bad review may be very different than yours.
6) Provide for syndication. I might want to take your reputation and make it my own and add more user feedback into it.
7) Provide for extensibility. I might want to take your reputation service and add my own layer of reputation service on top of it.