what does '!' mean in the rating?

Well, if you have not figured it out already, the ! button is for reporting discussions or responses that go against the myLot discussion guidelines.

If you read the guidelines, you will know what the + and ! mean. I think you shouldn't rate the discussion with -, it would be better if it's a spam you report to the Admin with the ! rated. If someone has a different opinion with you, just give your opinion and answer it. It will generate more replies to your discussion, but don't give - rating. He/she has helped you getting more responses to your discussions. Sometimes I give + for people who disagree with me and sometimes I gave them best response too.