When you communicate electronically, all you see is a computer screen. You don't have the opportunity to use facial expressions, gestures, and tone of voice to communicate your meaning; words -- lonely written words -- are all you've got. And that goes for your correspondent as well. 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.
In real life, most people are fairly law-abiding, either by disposition or because we're afraid of getting caught. And, perhaps because people sometimes forget that there's a human being on the other side of the computer, some people think that a lower standard of ethics or personal behavior is acceptable in cyberspace.
When you're working hard on a project and deeply involved in it, it's easy to forget that other people have concerns other than yours. You're taking up other people's time (or hoping to). It's your responsibility to ensure that the time they spend reading your posting isn't wasted.
Don't flame, or post-flame-bait. Be courteous!
You may not be judged by the color of your skin, eyes, or hair, your weight, your age, or your clothing. You will, however, be judged by the quality of your writing. Use proper spelling and grammar.
Pay attention to the content of your writing. Be sure you know what you're talking about; bad information propagates like wildfire on the net. In addition, make sure your writing is clear and logical. It's perfectly possible to write a paragraph that contains no errors in grammar or spelling, but still makes no sense whatsoever.
When someone makes a mistake -- whether it's a spelling error, a silly question or an unnecessarily long answer -- be kind about it. If you feel strongly about it, think twice before reacting.
If you do decide to inform someone of a mistake, point it out politely, and preferably by private email rather than in public. Give people the benefit of the doubt; assume they just don't know any better.