I can't really relate to the mindset of people who use downvoting as a 'I disagree' button.

That's a valid use of the button by design, HN is literally made to allow for that use. Plus it mimics real life interactions - there is a social cost/friction to saying things people disagree with or think are outright wrong. Most online chitchat places deteriorate because they remove such social frictions.

I would say it mimics real life interactions of some communities. I do not think that is universal. I tend to think that the communities, in real life and online, that permit civil discussion of dissenting opinions are the healthier ones.

I think there is a far greater real life social cost in violating standards of behaviour, such as aggressive engagement, or acting without empathy. I would argue that it is those influences that can be lacking in online discussions that cause them to deteriorate. There is also a lower barrier of entry for joining an online community than joining a real life community. A few dedicated but detrimental people can always evade safeguards and pollute a community to some degree, online communities being larger provide the possibility to each individual to do more damage, while also increasing the chances of there being individuals that would do so.