Key Takeaways:

1) It's easy for both users & site admins to manipulate ratings - be careful of drive by ratings or reciprocal ratings.

2) Be careful about negative ratings. Users may freak out.

3) Can show users only similarly-ranked users to complete with.

4) Reputation methods. Rating, ranking, favorting, rating raters, altruistic punishment.

http://www.lifewithalacrity.com
http://developer.yahoo.com/ypatterns/parent.php?pattern=reputation
http://www.rapleaf.com