Or simply force a comment if none already exist from a previous down voter. That way first one in has to give a reason for the downvote, which others can then +1 (or is it -1..)
It will still have the problem that child comments of a downvoted comment will not get as much exposure. It also means that even if the comment was just unpopular (despite being well thought out) it will be downvoted after the first person posts a comment disagreeing.
Another approach could be to factor child karma when ranking comments. Though this gets significantly more complex as there are many edge cases where this will not work as intended.