Ablogger and Internet entrepreneur Jason Calacanis coined the term "Internet Asperger's Syndrome" to describe the utter loss of all social rules and empathy that seems to hit some people for no other reason than that they happen to be communicating via keyboard and monitor at the time.
Calacanis figured out that people who do all of their communicating online wind up mimicking Asperger's behaviors because they are imposing the same disadvantages on themselves. In both cases, when the ability to see nonverbal responses and facial expressions goes away, so does empathy. Soon the thing you're communicating with isn't a person, they're just a bunch of words on a screen. A bunch of words that the little bastard didn't even bother to spellcheck.

#2. Low Cyber Self-Esteem (a.k.a. The Guy Who Everyone Hates but Who Never Leaves)

There's a place for everyone on the Internet to feel at home. When you can fill a message board with fans of The Wonder Years porn, there should be no such thing as an outcast. Yet, each forum, chat room or other online community seems to have a person or people who just don't fit in. It's not that they are necessarily horrible people, they're just the square trying to fit into the triangle hole. They get ridiculed constantly. So we've established that when you say, in person, "Jimmy, go away, you're a retard" that Jimmy is just happy that somebody used his name and acknowledged his existence. Even if the only reason you used his name was to tell him to go die in a fire.
The "we hate Jimmy" thread on a popular forum might be read by thousands of people. The attention-seeker gets what he wants, and the self-abaser gets an erection big enough to actually interfere with the signal on his wireless keyboard.

#3. Online Obsessive-Compulsive Personality Disorder (a.k.a. The Grammar Nazi)

We all reserve the right to mock people who post 500-word blocks of misspelled nonsense. But then you have the situation where somebody posts a perfectly clear and clever message but within their well-articulated points they dare to confuse "your" with "you're." And then somebody will flip the f*** out. Like a Mossad agent in rural America, you quickly discover that you've found a Nazi. Of the Grammar variety.

#4. Munchausen by Internet (a.k.a. The Sob Story Teller)

These are the people who lurk around innocently enough, and then, one day, tragedy strikes. Their dog, or parent, or maybe a close friend died. Maybe the poster themselves found out they have a terminal disease. You send this person your prayers and well wishes, maybe a few dozen kitten pictures and you hope they will get through it. Then, a few months later, another tragedy strikes them. Their best friend was raped, or paralyzed in an accident, or both. A few months after that, their father dies. Again.
"I can't wait to tell the Internet."
Soon it becomes apparent that they are either living under an ancient Egyptian curse, or they're making it all up.
It's so common that somebody else has already coined the sarcastic term for it: Munchausen by Internet.

#5. Low Forum Frustration Tolerance (a.k.a. The Frantic Browser Reloader)

This is the guy who makes a new thread, knowing he's just written the absolutely perfect post. A post that should be heralded across the Internet for its beauty, comedy and insight. It is such a good post that the guy is checking every five seconds to see if there is a new response. If he gets a response he quickly dashes out his own reply that will appear half a second later. If there are no responses to his perfect post then he will wait an eternity of five minutes before replying to his own thread with, "What, nobody has a comment? Helloooo???" You may also find this guy submitting stories to news portals like Digg and Reddit, losing an entire workday hitting Refresh (or F5) over and over, waiting for somebody, anybody, to digg up his submission.
There never has been an engine for instant gratification like the Internet. Our parents thought television killed our attention spans, but hell, with TV you still had to wait for the shows to come on, and they played at their own pace. On the Internet, the videos start when you f***ing tell them to. If they don't, off to another site. It's like a faucet: you turn the knob and you expect an immediate flow of lolcats.
It trains all of us to be impatient. And it's easy for the impatient to start looking at fellow posters or Diggers as just more pieces of content, morsels that need to be delivered the instant we want them. And why wouldn't we? This is a place where we can get a girl to strip for us on a webcam for like three bucks.

#6. Online Intermittent Explosive Disorder (a.k.a. The Thin-Skinned Rage-o-holic)

Like serial killers, these people seem pretty normal at first. For hours or even days, they'll carry on funny, charming conversations in a forum or comment section. But then something, anything, sets him off and he devolves into a tantrum that would make Christian Bale say, "Dude, calm down! Jesus."
First, there's the obvious: Most of us suppress our real-life spurts of rage for fear of getting punched in the face by the person we're screaming at. Second, on the Internet, where your looks, job, income and fancy clothes won't buy you any respect, some people seem to think they have to protect their reputation like an old west gunslinger: shooting down anybody who calls them out.
But then there's the third, and least obvious reason, which is that without tone of voice and body language, it's hard to convey mere annoyance or mild anger, without the fear that the person you're conveying it to just plain won't notice. So they think they have to crank it up to a 10 every time they're crossed, even if they don't mean it.
That's probably the weirdest part, that these people who are SCREAMING INSULTS IN ALL CAPS are often at the same time sitting in a cubicle somewhere, sipping coffee and conversing pleasantly with the person next to them.