It would be nice to be able to vote down comments.
Sometimes I find users leaving very unhelpful one sentence complaints about questions and/or answers, but it is not nearly serious enough to warrant a flag. I think such comments should be discouraged, and it isn't always enough to discourage them by leaving a respectful comment of your own. People often feel justified in their complaint and the way they've voiced it, and won't see why their response is unhelpful. Allowing the community to have a say in which comments are helpful and which ones are not might help this.