It's ironic, really. Computer networks bring people together who'd otherwise never meet. But the impersonality of the medium changes that meeting to something less -- well, less personal. Humans exchanging email often behave the way some people behind the wheel of a car do: They curse at other drivers, make obscene gestures, and generally behave like savages. Most of them would never act that way at work or at home. But the interposition of the machine seems to make it acceptable.
The message of Netiquette is that it's not acceptable. Yes, use your network connections to express yourself freely, explore strange new worlds, and boldly go where you've never gone before. But remember the Prime Directive of Netiquette: Those are real people out there.