I think this is what has been the most damaging aspect of Reddit's growth. People don't see the up and down arrows as a measure of quality, they see them as a source of agreement.

Yes. The most frustrating thing about this site is getting downmodded (repeatedly) without any replies. If you don't agree, say so! Then maybe we can have an intelligent discourse. I have often made comments that are counter to the "groupthink" of the thread. Sometimes because I disagree, sometimes just to play devil's advocate to see where it leads. But always to learn and share.

If clicking the down arrow without replying becomes a trend, then no one will want to disagree. Then where will we be?

I agree 100%.