I like the idea of deducting karma for a downvote. However, forcing a comment on downvote, while perhaps leading to less downvotes, might also just lead to many more comments that are not thought out and simply put there in order to downvote.

Maybe we need more nuanced voting options? While this could get a bit complex, such a system may function better. People could upvote a post as it is well written/thought out and/or is provocative/clever/interesting, but downvote it as they disagree. The overall comment karma would balance out, rather than it just being downvoted as lots of people disagreed with it.

Facebook is now using a similar type of system with their multiple emotes. I think such a system would work well with the HN community. It may lead to more thought out comments, as simple opinions can be expressed more accurately via the voting system.