Ratings are worthless without trust. Actually PeerMountain is building a decentralized trust protocol that supports this. They call ratings "attestations". It's worth a read.

Interesting.

Do they have the concept of "transitive trust"? (Is there a better term for that?)

Let's say I want to buy some new camera. I'd like to express something like:

- People on HN, I trust for 80% (say, because of the technical nature of the product)

- Friends on facebook I trust for 40%

- Friends of friends on facebook, I trust for 10%

Then using this spec, I'd like the system to come up with a sorted list of cameras, with ratings. Would this be possible without privacy issues?

By the way, if a rating system turns out to be robust, we could potentially use it for rating politicians :)

Yes. Transitive trust is a core concept for Peer Mountain. And they call it exactly that.

Interestingly enough I'd trust a long, well written blog entry as a rating much more, than I trust current amazon reviews.