The voting system should be at least a partial feedback loop against that effect, but it's obviously not perfect, especially if people view it as "agree/disagree" rather than "quality/not quality". I get the sense that reddit has historically been the former whereas HN was mostly the latter.
It's actually the opposite, historically rediquette was to upvote quality and not downvote just because you disagree. Where as HN policy is to downvote things you disagree with. The difference is just that reddit grew faster and had a broader focus. Without careful moderation HN will grow (both in users and topic types submitted) and will suffer from eternal september as well.
No, but it's pretty much bound to wind up that way. The effort to put forth a well-considered, nuanced, reasoned point of view is an order of magnitude more than that required for a snarky one-liner about "dude, like, the US is, like, totally not a democracy and stuff". Which means that the latter out-competes the former.