If someone actually uses the Thumbs Down button in a forum thread, that person usually gets a number of PM's asking why, a few "disciplinary" thumb downs and usually a whole spiel about it not being done here, against the UNwritten rules, etc.
Thumbs are for opinions (IMHO). You like something, agree with it or are just generally favorably disposed toward it, you give it a thumbs up. If you don't like something, disagree with something or are unfavorably disposed to it, you thumb it down. If the site creators didn't want us to use thumbs down, they would not have included it - but there is both a Report Button AND a Thumbs Down.
So perhaps a section in the FAQ that delineates, very clearly, what the site creators idea behind the Thumbs was, and examples of both improper and proper use, would be helpful.
And if I am wrong, and the creators did not want us to be free to express both positive as well as negative opinions, perhaps they could follow Facebook's example and just go ahead and remove the Thumbs Down button so it ceases to be such a big issue.