Upvotes should be for 'contributes to discussion' not for 'I agree'. But it's universally used an agreement/disagreement button on Reddit which amplifies the hivemind massively.
That plus the fact that downvoting is only granted once you've spent a considerable amount of time participating in the community can have a huge impact on the nature of discussions.
> Which is mostly the opposite of HN
You must be new here. :)
While I can definitely see a lot of people trying to show off how smart they are on HN, they are doing it through demonstrating things and making solid arguments. On Reddit, that is usually done through personal attacks and demonstrating themselves.
So many times I went on reddit and saw someone making a solid argument, only to be derailed by someone going through their comment history and bringing it up as a "aha, gotcha" point in an argument. Even in cases where it shouldn't matter, something like seeing a poster getting downvoted to hell just because one of their comments wasn't critical enough of Trump in an argument that had nothing to do with politics and was purely technical.
Which is mostly the opposite of HN, which skews older and actually more educated, comment guidelines are much stricter, and the topics covered are much more technical and narrow. Even non-tech topics covered on HN (e.g., in-depth discussions of particular judicial rulings) aren't really of much interest to an average teenager or a person in general. Which leads to a few different potential effects:
a) Older and more educated people have much less reason to "flex" how smart they are. The also have fewer occurrences of delusions of grandeur/Dunning-Krueger effect.
b) Niche technical topics have a barrier to entry, which already eliminates a gigantic portion of the toxic audience. The topics themselves are also largely uninteresting to an average person.
c) Stricter comment moderation eliminates a lot of potential toxic material, like every other comment devolving into a shitty attempt at a joke or a personal attack.
And for me, personally, the biggest difference seems to be that on HN, there is a way higher prevalence of people trying to figure out what is right, instead of who is right. I am glad to be proven wrong in an argument here, because it gets me one step closer to finding the "most correct" answer. And the arguments themselves (usually) tend to be very polite. While on reddit, it seems like the vast majority (outside of some specialized and fairly niche subreddits) is just obsessed with proving that their answer is the "most correct" one, so they go all out on the means to reach that goal, which tends to any sort of arguments to become very uncivil very fast.