That's what Jason Calcanis calls the lack of empathy, the failure to acknowledge common humanity, that he sees too often in the online world. And yes, he knows that this is an insult to people with Asperger's. See his links and also Catarina Fake's reflections for further details.
Whenever someome raises these concerns, there are always plenty of people who show up and say "What, can't you take it?" or "You need a thicker skin." What these comments tend to miss is the fact that participating in online discussions is almost always a voluntary activity. So sure, most of us "can take it" — the question is, Why should we? What value do we get in return? When you blog and welcome comments, you're hoping for constructive and interesting ones, and if you get too high a proportion of belligerent and dimwitted ones, you're likely to consider disabling the comment function. And why shouldn't you? Nobody has an obligation to interact online, much less to do so through the one medium of blog comments. (In fact, there are some people who think that you can create better conversations by using your own blog to reply to what people say on their blogs. Kinda like what I'm doing here.)
So, what counts as "too high a proportion of belligerent and dimwitted" comments? There's obviously not a one-size-fits-all answer to that question. I've been amazed for some time at the levels of hostility Megan McArdle is prepared to accept (though lately she has been more active in moderating than she used to be, and that's had a real effect on the conversation). Over at my other internet home, The American Scene, the general tone of comments is milder, but there's still too much wrangling, sneering, and mocking for my taste. I've stopped subscribing to the comments and am less inclined to visit the site at all. It's not as pleasant as it used to be, and — maybe this is a function of age — I don't see why I should expose myself to more unpleasantness than life is already prepared to deal out to me.
Note that I'm still enabling comments on this blog, though. Maybe that's because I don't get too many. . . .