Problem with negative Karma, is that people like Chaosnaska could give negative karma to anyone for no reason at all. Granted, I hate him and hope he gets banned instead. On numerous occasions has he backpedaled on what he has said and is a hypocrite in every sense of the word.
Another problem with negative karma, is that it would be abused for rediculous reasons like "this person doesn't agree with me" or "doesn't believe in god" or some stupid shit like that.
Please don't implement negative karma. Just see Xbox Live for why this system fails. People will spam it because they think it is funny and use it in the wrong way.