That negative karma thing sounds like a good idea in one way, but a very dangerous one in another. Perhaps something like that should be done "in private" by Ubuntu, with negative karma something entirely separate from positive, and which only Canonical employees are allowed to assign and view (that is, that the people with the negative karma and all other non-employees are not even aware of it). That way, they would get some idea of where things are going wrong, but without upsetting their user base.
Who knows, perhaps they are already doing this? :)