Self-righteousness and emotionalism. I've argued with plenty of people on the right, and however fixed in their positions they might be, they were always willing to keep debating or, barring that, to shake hands, agree to disagree, and move on. They recognize that we're both just people trying to find the right answer, and they realize that reasonable, well-intentioned people can disagree about what that is.

Most leftists with whom I've debated in real life (most - not all) go into the debate with an extreme self-righteousness: they assume that they are the defenders of the poor and vulnerable, and that everyone who opposes them is just rationalizing their prejudices and self-interest. They resort to emotional non-arguments ("I've seen poverty - don't tell me that it isn't capitalism's fault!"), attempt to disqualify their opponents ("You're white, so you aren't entitled to an opinion on a racialized issue like poverty"), appeals to mob mentality ("You're the only one who thinks this - everyone I know realizes that capitalism is exploitative"), passive aggression ("I believe in socialism because I actually care about people other than myself"), threats ("If you keep invalidating my experiences, I swear to God I'll tear your fucking throat out"), insults ("You're a stupid fucking cracker"), and, finally, flaming out of the debate ("I don't want to talk about this anymore. Hopefully you grow up and learn a little about the world").

All of these are things which I have actually been told in debates with leftists on a university campus, or on other prominent university campuses (the threat, I'll admit, was something that I observed a leftist tell my friend at a debate tournament at Harvard a few years ago). These are attitudes and expressions which aren't just uncommon on the left: they're acceptable, viewed as legitimate, and will even enhance a person's social status. All of this taken together forms the worst part of leftism, which is the constant virtue-signaling and posturing (everyone wants to one-up everyone else by pointing out how they need to check their privilege, or correcting the ableist connotations of their language, or whatever other pedantic circlejerk they get into).

You can find some of this on the right: tendency towards emotionalism, for instance, is rampant on all sides. But you don't find it expressed in the same hostile, self-righteous terms that you find on the left. Even leftist intellectuals pour scorn on how stupid and corrupt their "Koch-fueled, white trash" opponents are supposed to be. This is worst on college campuses - a conservative speaker stands a good chance of being disinvited, and is almost certainly going to encounter hecklers and protestors. It's inconceivable that a liberal speaker will deal with the same sort of vitriol.