In the case of digg/reddit the payoff is interesting content. It would be a harder sell in this case because looking at commercial sites is usually less interes

In the case of digg/reddit the payoff is interesting content. It would be a harder sell in this case because looking at commercial sites is usually less interesting especially if the quality is mixed. If you implement a mechanism where the best sites float to the top, well then the sites which need criticism the most (coincidently the least interesting to review) never get it. I think it's a good idea but you really need to think out the incentives more thoroughly instead of just hoping for good-will and communal feelings to keep it going.