A few months ago, I posted this question on the CV meta asking about why a flag for deletion that I added to a comment was declined. This question generated what I think was some very interesting discussion. In particular, @NickCox made an interesting point that there is no way to downvote a comment as a way of saying "this comment is inappropriate and/or does not add to the discussion", in contrast to how questions and answers are handled in the StackExchange model.
I'm actually somewhat surprised that comments are handled so different than questions and answers. I personally would appreciate a way to indicate that I think a comment does not add to the discussion without outright flagging the comment for deletion and/or responding to the comment (responding to some comments can sometimes take a thread off-track, so in some cases I would prefer to have a downvote feature rather than posting a new comment).
So, my question is, is there any reason why downvoting is not enabled on comments, and if there isn't, could downvoting on comments be implemented on CV and other StackExchange sites?