I would assume a post rating feature would get more use than a thread rating feature -- because some indivdual or clique will more easily be able to target a message about which FTers they like and which ones they despise and pile on in that eay; and because the post rating (and "like" is a kind of binary rating) would be more visible and send a more targeted message than a multi-star rating of an active thread with differing opinions in it from lots of different people.