This is a legitimate and rather frequently made suggestion, but a contrary suggestion was made in a comment by pg

http://news.ycombinator.com/item?id=117171

where he wrote, "I think it's ok to use the up and down arrows to express agreement. Obviously the uparrows aren't only for applauding politeness, so it seems reasonable that the downarrows aren't only for booing rudeness."

On some threads, I can find occasion (as I do here) to post a reply indicating substantive, polite disagreement with a previous comment, to which I reply with my disagreement. On some other threads, I see comments to which it would not be good use of time to post a disagreeing reply, and sometimes in those cases (not all the time), I find it most appropriate to downvote to indicate disagreement. I can tell that I am not alone in that practice.

What I especially like to do is to upvote any comment that is good, where "good" is defined as helps me learn something I didn't know or makes me think from a new point of view. People can disagree with positions I hold and still get upvotes from me, especially if they post verifiable information indicating why they hold the positions they hold.

I'm fairly happy with where the quality of discussion is now, and I'd like to see it continue at the same or better level. I think a guideline that encourages conscious and non-reflexive voting based on quality rather than agreement would help, even if the other only difference is that people think more about why they are voting as they currently do.