We can vote up and down for Questions and for Answers, but we can only vote up for comments. As some comments are worthy of down votes, this is a feature suggestion to add the ability to vote down on comments.
One question for this feature though: What should the minimum reputation level be for a user to be able to cast a down vote on a comment, should such a feature be added? I'm thinking that it might be wise to have a higher reputation requirement than for voting up.