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.
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.