I recently happened to be on the site while I wasn't logged in, and I noticed that on each post, where the comment link normally is, is a feedback link. When you run your mouse over it, it changes to the question, "Was this post useful to you?" followed by yes/no buttons.
If someone clicks one of those buttons, what happens? Does it count as an up- or downvote? Does it flag the post? I can't see anything in the UI which would seem to be the result of one of those buttons being clicked.