In popular reddit subs, you can't have an unpopular opinion. Your link submissions and your comments get downvoted to oblivion. It also happens in less popular ones.

Reddit reacts incredibly negatively to poor tone. You can be completely right about something, and unless you're nice about it, you will be below threshold in no time flat.

Most of the comments I see hidden on Reddit are there, not because of simple disagreement, but because the person posting comes off as haughty or smug.

I can't complain too much. Anything worth saying is worth saying nice.

Dunno, I've found Reddit seems to upvote controversial opinions much more frequently than HN. HN definitely filters out anything that is out of line with the consensus...

By volume, I would say Reddit comments are used for cheering.

Of course it does, in part thanks to the moderation and flamewar detector. Read this thread, especially PG's comments. https://news.ycombinator.com/item?id=6596038