I think such a thing would probably get used in personal fights, where people who don't like each other go about rating each other's threads down, rather than basing it on what is good content or not. The upvotes might be used in a similar way, but forums like the Minecraft forum do have upvote buttons for posts, and I think they might be a good idea (but they would be mostly just for fun).
A downvote button would just a cheap way to publicly say "I don't like what you wrote" without stating a reason, but if posts actually go against the rules there's still a report to moderator button. I think that button is better than a downvote button.