I almost never downvote. The only time I downvote is if something is extremely inappropriate, negative, trolling, or is degrading (IMO) to the quality of this community. I never downvote when I disagree. If I disagree, I reply with my disagreement. I wish others would too. Not much to learn from downvotes. Lots to learn from discussion and healthy debate. I upvote often, to anything that I think adds value.
I often hear people advocate a policy of only downvoting offensive or trolling comments on news.YC. I think it's a bad idea. My goal in voting is always to improve the experience for the next reader. That means that I vote up what I think is the most interesting or important information, and vote down distractions and annoyances of all kinds: not just trolling, but also poorly thought out arguments, boring observatio…
This is quite a debatable flaw in your argument. Worth a downmod? Hardly. I am pointing out the obvious to some (or many), but to others, and those who share your viewpoint, this might be a blind spot, or simply preposterous.
I think a community like this (HN, that is) is better off just getting into the habit of good discussion, such that it becomes implicit in the culture (which kind of is already). Just make sure the SNR is naturally high. That's quite a feat.