I doubt that you can really influence the depth of a debate with any voting system. Voting is good for filtering large amounts of information. For everything else we'll have to use our brains for the time being. I agree that fear of getting downvoted is a very bad motivation in a debate. Downvotes are just unnecessary in my view. They only create bad blood and don't help to filter information. I've decided to never downvote.
But, you know, the day I really start to worry about downvotes is when the karma to $$$ conversion rates are announced ;-)
I agree that downvotes are much less useful than upvotes. That's why there are so many restrictions on them. There are no downvotes on submissions; you don't get a downarrow on comments till you get a certain karma threshold; and even then downarrows are missing in some places.
But, you know, the day I really start to worry about downvotes is when the karma to $$$ conversion rates are announced ;-)