Be very careful of the design of rating systems and reputation systems. They are extremely difficult to design well, as they too often can be gamed, or fall into reciprocity such that they are meaningless. My personal advice is just don't do it at first -- save it for a 2.0 version of the product, not 1.0 beta. If you are going to do it now, really study it -- there is a lot of good academic research on issues of reputation. It can be hard to slog through but it is worth it. Offer a grant to Danah's school for them to do research for you on the topic. Endorsements are a best way of doing reputation for now. They is also is imperfect and vulnerable to reciprocity games, however, as least you can see if two people are playing that game just by looking at the endorser and endorsee. If you find too much reciprocity you can basically ignore both players.