IM and SMS aren't public forums where people you don't know can barge in with their own take on what you just wrote, though. IRC can be, but it would be very difficult to have a grasp on every IRC conversation happening everywhere so you could respond to something you find incredulous.
Unlike Twitter, which incentivizes people carrying on infinite numbers of asynchronous conversations, and encourages people to find them by bringing up 'interesting' ones on the timeline.
I think you've found the real problems, none of which are "short form" messaging: I agree with you that there's an aggression to a public forum where message can be commented on by everyone and every past message is "fair game" for "fresh" conversation. Those do seem to lead to plenty of aggression.
In the heyday of blogs, long form posts weren't immune to those effects either and that's why blogs invented things like spam checks and comment lockdowns and comment moderation tools.
I think there's a degree of civility brought about by IM, SMS in particular as you normally know the person, or have interacted with them long enough so that you consider them (and they consider you) part of your 'circle'.
IRC - on specialist forums perhaps there is some civility because everyone in that forum is likely experienced and interested, so mutual respect and good manners reign.
But when you put together a heterogeneous group of people with no defined clear goal for the group and mix in anonymity by default, you get 4chan.
It'd make an interesting sociological study, which factors (privacy, anonymity, interest group, etc.) correlate with aggressive online social groups.
Any system with upvoting leads to this, particularly after a certain threshold of diverse viewpoints. Too much attention is needed to accurately defend your viewpoint, leading to feelings of social isolation, outbursts. This can be exacerbated further if your outbursts wind up being popular, rewarding you with social bonding.
This also tends to remove a bunch of nuance from the most popular discussions. Nuance adds more noise, and people have no remaining attention for it. This lack of nuance feeds back in to general resentments.
100%, it's always these brownie points, it's like the currency of the virtual world, eventually people will reference and base everything about it, and will do anything to have a "ratio" in a form of wining. That's why -and as much as it sounds absurd-, I like the digital interactions with no upvotes/brownies, like anonymous boards (4chan etc.) or just usual chats.
200-odd characters is simply not enough to fully articulate some thoughts and is contrary to how people communicate in real life.