1) People down vote to express disagreement.
2) People down vote to eliminate value-less comments.
There needs to be a separate function for each of these. Perhaps the current down arrow for disagreement and some sort of irrelevance flag which wouldn't impact karma, but would still make the comment fade or eventually disappear.
I typically approach down or upvotes to my comments as (dis)agreement.
What is important to me is furthering the discussion if someone downvotes me for disagreement.
Requiring a dissenting opinion with a disagreement downvote would not only satisfy my wanting to hear the objections to my comments, but it would also require the downvoter to have a good dissenting opinion (lest his post be flagged as valueless/irrelevant).
Requiring a dissenting reply with a downvote is for my own personal reasons, but I do think there needs to be a distinction between disagreement downvotes and irrelevance/valueless downvotes.