Everybody should be familiar with this issue. When I used to go to conventions at Toronto Trek (up until about ten years ago) one of my favourite activities was to participate in panel discussions. Said panels tended to be more focused discussions than the multitude of them that cropped up in conversations throughout the convention and particularly in the lounge areas set aside for having a refreshment, a snack, maybe watching something on a TV or film screen and/or just hanging out with fellow fans. But one thing that was common in any discussion, no matter where and when it took place, was the common courtesy everyone seemed to have for others. I saw very little to no immediate assumptions about others if they had an opposing viewpoint. Everyone got a chance to speak and be heard and without getting shouted down, criticized and/or insulted. This was the epitome of social discourse. And this is pretty similar to the discussions we see in everyday life whether it be at work or out in public. In the real world, at least in terms of everyday discourse, more people seem to have a live-and-let-live attitude (of course, there are always exceptions). But there are two circumstances where this civility has a tendency to evaporate: politics and the Internet. Maybe people take their cues from politicians and media commentators where nothing seems to be off limits to say. Maybe it's different in private, but in public forums and venues politicians, and those who support them, are not shy about making truly ridiculous assumptions and accusations and saying them aloud. This gets carried into discussing practically any kind of subject on the Internet: the filters are gone. People say things (bravely) online that they most likely would never dare say face-to-face. It's true that unless you're using Skype and chatting face-to-face online you often can't pick up nuance in text on the screen. You can't read facial expressions and body language and tone of voice which all underline how we interpret what someone is saying to us when face-to-face. As such many people can be very quick to assume the worst and make crazy assumptions about another person. In person when faced with something someone says we disagree with we're usually inclined to counter simply with, "Well, I don't agree..." and proceed to explain why. But online it's often, "you really are stupid." or "fucked up" or whatever other put down they can think of. And this isn't just on message boards. I see it frequently when people post comments regarding news articles in online newspapers. It can quickly become downright viscious and demeaning. In day-to-day life we strive for a world of civility where we try to deal with disagreements reasonably before they can escalate into arguments and open hostility. And while there is indeed civil discourse to be found online I think that lack of civility seems to have the upper hand. Why is it so different? Why can we give another person the benefit of the doubt more easily in person than we can online?
If some people behaved in person the way they behave online, they'd get their asses beat. That's probably reason #1.
Or perhaps it's a case of safe behind our computer screens we can say what we really mean. When speaking to someone face to face we can all wear a mask from time to time. The flip side to the argument is that people who are shy can open up a bit more on the Internet than they would in person. And of course as you point out written text lacks all the nuiances found when speaking to someone either face to face or over a phone. One other point to consider is what the Internet actually enables us to do, speak to people who we likely would never meet otherwise. You might live in area which leans to one side on the political spectrum so there is a higher likelyhood of coming across people who share similar beliefs, however due to the Internet you can come across people who hold the opposite of your views, and lets be honest here how many of us never thought that a person is an idiot/stupid etc.. for holding a certain view. What prevents us from saying that face to face as you say the rules of civillised soceity and perhaps in some cases the other person is bigger/stronger etc...
I know that it's easy to assume the worst of another online particularly in a world where we can be fearful that anybody could be out to screw us over in some way or other. And while one must have a decent measure of awareness when dealing with others (particularly those you don't know) I don't personally subscribe to the assumption that everyone you meet is most likely out to get you or rip you off. Even in misunderstandings anybody can make mistakes. If we hope that others can be forgiving of our own honest mistakes then we should be willing to be forgiving of the honest mistakes of others. Sometimes we say things that don't come out right and the way we intended. That can certainly happen (and has happened) to anybody and everybody. It's true in real life we can still encounter people who are quick to take offense and immediately assume the worst, but usually we have the chance to correct or explain our error before things get out of hand. But online a lot of people can be very quick to judge and and attack.
Maybe instead of being critical of people who take offense, you stop to consider why they are offended in the first place.
It doesn't jibe with human nature to assume that people are always civil in person and that peoples' bad sides only come out online. Remember war, crime, civil court, why we need police and armed forces? If people were rude only online we'd have none of those things. Although assault is a crime, and anyone literally beating up someone at a convention would themselves go to jail and face felony charges, what Robert Maxwell said about getting beat points to the fact that in some venues the costs for acting out are higher than in others. Additionally, though there are exceptions, I believe that, as a rule, people who go in person to an event such as described in the OP have made an investment of time and effort, not to mention the entry fee, to be there for positive reasons. Those costs mean that someone will have to work harder to be disruptive. Of course, costs aren't perfect filters against disruptive behavior, because people willing to advocate, who believe in things strongly enough, and who have the resources will pay any price to advance their causes. We've seen that in Gamergate, when it actually comes down to the conventions themselves banning certain groups; they'd've come to be disruptive otherwise. So, I don't believe that framing it as face-to-face versus the Internet is the right way to look at it. Message boards are free to join and comments are free to make. TrekBBS is well-moderated, but not all of the Internet is, clearly. So, there are fewer filters inherent to the medium of the Internet, compared to, say, being at a convention. By the way, just to dispel any question, I'm not advocating fees for message board use. There's no subtext of that in what I'm saying. Effective moderation is the effective tool. Generally speaking, we need police in the real world and moderators on message boards. They serve similar functions, to keep things from getting out of hand.
Yes, your explanation is basically what I was getting at: there are higher costs (monetary and otherwise) to engaging face-to-face than there are to posting online, and people behave accordingly. Online comment sections are almost universally garbage because of this: it takes a few seconds to pop off some stupid shit on YouTube, and nothing bad will ever come to you because of it. Shouting the same thing in the middle of a classroom, church, or a convention panel would be received rather differently.
Anonymity is necessary for asshattery. Security too. When in the physical presence of thy foe, there is a tiny part of your brain reminding you of the chance, no matter how miniscule, that they might be able to kill and eat you.