When somebody gives a thumbs down, give them the ability to explain why with a comment.
On our other docs sites, we find that's very useful to give users the opportunity to comment whether it's a THUMBS UP or DOWN. We find out a lot about which docs users find the *most* valuable and also what doc trends differentiate us from others in a good way -- when users comment with a thumbs up (which they often do!).
Any timeline as to when you might be releasing this feature?