I have seen this work very well on other forums. To prevent spammage, sometimes users are given a limit of how many times they can use the feature per a set amount of time. However, like karma, it would also work if there was simply no thumbs-down option. At the very least, allowing a positive-feedback-only system would still remove a lot of need for posts containing simply "i agree", "this", or "QFT". Aside from that, I think most people will want to elaborate when the particularly disagree with something -- and if someone has already elaborated on why a particular post is disagreeable, they have the option of "liking" that person's post, anyway.
Anyway, good idea, Dema. I like it. (:
(thumbs up)