There is a rating option at the bottom of the post your viewing, you can use it to vote for how good or bad you think that post is. From what I can see, depending on how many people have posted decides if a rating is show. For instance if you rate a post that has 100 views and no one else rates it, the rating doesn't show. If you rate it to begin with (and others too rate it) then the rating will show up (But you need to have the rating done by at least 2 or 3 people from the threads start, and before a large number of "Views")
