I'd like to make two points about this:
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1) I do not understand in any way the sentiment that discussion should ideally consist of people agreeing with and encouraging each other.
The reason I speak with people is either to inform or learn. If I'm informing, this is not really a discussion. I'm just telling people something that they may not know. There are two ways to learn: one is to listen and not speak, which is the mirror of the above. The way to learn through speaking is that somebody says something, I dispute or question that thing, and that person shows me why I'm wrong.
So the way to learn while speaking is that someone says something, I say something negative about that thing, and then that person says something negative about the negative thing I've just said.
Friends and family are what you need, not the empty, uninformed, ritualistic, and above all socially-pressured positive comment of strangers.
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2) Following up on the first point (which is mainly a personal observation), it's important to say (although completely unsurprising and obvious) that negativity is relative. On HN (or reddit, or any comment site), the first post or OP is assigned positivity.
A great example is this very OP, which is an accusation of negativity on Hacker News, which with no context is quite obviously a post with negative sentiment. The way you would grade reactions to it in a vacuum would say that other posts disputing its conclusions are positive.
Its methodology, however, requires that the posts disputing it be graded as negative: "Criticism of technology, skepticism toward announcements, complaints about industry practices, frustration with APIs." What are the OPs that contain these posts? Links to technologies and technology advocates, announcements, recommendations of particular practices, descriptions of APIs.
Accusations of negativity and tone policing are always content-free social control. If I set my point of view as positive and uplifting (while posting about how HN and reddit and social media are evil poison promulgated by evil people who should be physically stopped to save civilization), I can silence dispute through calling the mods rather than through discussion.
The more indefensible my position is, the more I will prefer the sort of "discussion" where I say something, other people dispute it, and I accuse them of being negative people with implications of bad faith and possibly psychological unsoundness.