(Not all of that impression; certainly upvote-downvote systems tend to exacerbate groupthink. Even so.)

* Twitter in particular has the problem that if you want to interact with the other side, you have to pick a particular person to interact with; yet having someone you don't know initiating conversation just to disagree with you is rather unpleasant, especially if you're "famous" and end up with a lot of such conversations. On Reddit at least you can address an entire subreddit and avoid targeting anyone personally. You will probably be downvoted, which isn't great, but...

Eh why is that exactly? This is pretty much like calling people who object to the ideas that others present on Twitter as harassment. When you go on a public forum and present your ideas you open yourself to a public debate, hearing things that might be "unpleasant" should fall well within the realm of expectation in this case.

But the main one is the blurred line between public and private. Posting here definitely comes with the expectation of a public debate and so people who aren't in the mood for it will not post. But on Twitter everyone is encouraged to publicly post everything from random status updates to highly personal matters to idle chatter that is meant to only circulate among friends (and may not be fully thought through). You can avoid this by setting your account to private, and many do, but this blocks the spontaneous spread of posts through the social graph by means of retweets and replies, which is IMO the best thing about Twitter. There is no granularity, either: only one account toggle between all your tweets being visible to the entire world or to just your direct followers. (Including replies, which makes private accounts an imperfect shield: if you have a private account but chat with a public account, people can still see their replies to you and infer the content of the conversation, plus your username.) Also, whereas you can easily leave HN or Reddit or some forum altogether if you get fed up with it, leaving Twitter may be tantamount to losing contact with friends; you can make your account private but it's not the same sort of mental break.

The worst arguers on the Internet don't just disagree; they bring a moral judgment along with it. "You're wrong for believing in X, and the fact that you believe in X makes you a bad person." If you have enough people who agree with this, (and this often has absolutely nothing to do with broader support for X or whether X is actually bad or not) you actively feel hated, demeaned, etc for believing in X and voicing your opinion. Even being polite will provoke an enormous amount of vitriol simply because it's very easy to lump in a polite X-believer in with preexisting ideas that all people who believe X are assholes.

A lot of people react to this with, "Well, good. There are a lot of shitty Xs that people believe, and people really should be afraid to say those things out loud."

That's all well and good for some opinions; I'm not advocating being polite to, say, neo-Nazis who are denying the Holocaust or advocating genocide. But people rapidly expand X to "whatever I disagree with," and the result is that we treat disagreement on legitimate, reasonable issues that people will naturally disagree on as equivalent to endorsing genocide and believing that the Holocaust didn't happen.

It doesn't help that the Internet naturally magnifies hatred because it's very, very easy for 5,000 people to get ahold of something and leave one really nasty comment apiece. They spend 10 seconds writing a comment, but the person who originally posted X gets 50,000 seconds of written vitriol. It's overwhelming.

As a result, there are certain issues on the Internet (and in real life, for that matter) - vital issues, important issues, that I will not touch because they are far too likely to dump a massive amount of vitriol on me. I have better shit to do. As a result, the best possible outcome for me is not to engage entirely.

And then those exact same people who dump anger and hatred on all believers in X complain that said believers in X are "avoiding them." Well no shit they are, who would volunteer for that?

This is the core problem: what is "legitimate and reasonable" and what is closer to neo-Nazis (to pull your example)?

Painting a target on myself here for the sake of illustration: I am a Christian, and I believe in the literal truth of the Bible. This means, for example, that I believe homosexuality is a sin. (which, for the sake of clarification, does not mean that I hate other humans who are gay).

Where does that put me on the spectrum from "legitimate, reasonable issues" to "neo-Nazi"? The answers to that vary far and wide, and the way I get treated online varies accordingly.

I've been treated as another person at the conversational table ("so why do you think that?", "what does that mean for how you treat others?"), and I've been told I'm "objectively a terrible person" (and much worse) for believing what I do.

The problem is often less "how do I treat people who have reasonable disagreements with me" and more "what constitutes a reasonable disagreement?"

"Hey Bob, here's why you are wrong."

This is bad:

"Hey everyone else, here's why Bob is wrong and evil."

Twitter and the like are mostly 2 masquerading as 1. Call it open debate or whatever, I still don't like it.

Without that hard, often-gated work, how is anyone supposed to form an independent opinion on something like global warming, models of crime, economic policy, intimate relationships, pedagogy, and so on? So what you're seeing is not the discussion of ideas, but rather the negotiation of clan policy and identity.

There's no policy that you can implement to stop this "echo chamber" or "groupthink", which are merely symptoms of a deeper issue: that the antecedents of independent opinion on a wide range of phenomena are simply not there, and that obtaining quality is resource-intensive with non-trivial opportunity cost.

I truly miss the open forums (and even IRC, although that tended to be more focused) of the late 90's early 2000's yeah they could get vile some times but that's a bargain price for getting a public forum in which anyone can express their mind no matter how "wrong" or "offensive" some one can perceive that opinion to be. Today we get people claiming getting PTSD over twitter and every online community adopting a COC which often leaves no room for discourse that would make some one uncomfortable all and all forgetting that ideas that make some one uncomfortable are the ideas most worth discussing. 60 years ago it was "uncomfortable" for way to many people people to hear that blacks should be allowed to use the same bathroom as they are and thank cthulhu we didn't had Twitter and Reddit back then.

An obvious first step is removing downvotes so you can't completely obliterate someone who crosses sides to try to have a conversation, but after that, what else would help?

In my mind, some kind of code of conduct would help further those conversations, because it doesn't prevent people from expressing their opinions, it just makes them express them civilly. And if they express an opposing view without being overly antagonistic, then hopefully the other side wouldn't start insult slinging, and if they do, then that's what the COC would be there for.

I think there's a difference between having a "wrong" or "offensive" opinion, and expressing it in an offensive way. Do you think there's a way to achieve that balance, so that people can feel comfortable to express their opinions on either side, regardless of right or wrong, without it devolving into mudslinging and pure animosity on both sides? How do we accomplish that?

People mistake being challenged for being offended, having a discussion especially on a polarizing subject isn't supposed to be comfortable, but sadly we have too many people that have been cuddled to believe that it's their right to only hear the choir of angels reinforcing their opinions.

Heck even if some one decides to make a comment about ones maternal employment preferences I would not see a reason to moderate that, because unless you are discussing prostitution it most likely has very little to do with the actual argument and so would not score one much points in a debate.

Skilled debate in a venue like Reddit requires an understanding of esotericism, of not simply laying out facts and implications based on your own assumptions, but presenting a fascinating puzzle to the reader that leads them to challenge their own assumptions without being challenged by anyone in particular. When successful, such a puzzle glides beneath the surface tropes of the discussion, presenting the perspective without provoking hostilities. The karma system does not reward this very well, as those carefully crafted puzzles tend to get middling scores, while simple agreement, rationalization arguments(why your assumptions are right and the critics are wrong, from Someone Smart), and congratulatory joking trigger the instant upvote response. But putting in the effort into that dialogue is essential to engaging the community.