Here's something that leaves a really unpleasant taste in the mouth: the harassment of Kathy Sierra. The comments that were posted about her were not only of an extreme nature, but seemed to hinge especially on the fact that she is a woman in the world of information technology. As a female web developer, this scares me. It's bad enough being hassled for being a geek, without people hassling us for being girl-geeks as well. The comments and threats made against her were completely unacceptable.
I don't know if the answer is a code of conduct for internet use - the problem is, it should be completely obvious how to behave on the internet (hint: it's the same rules as face-to-face interaction) but despite all the rules and guidelines you see on forums and mailing lists and the obviousness of how to behave, you still get the occasional troll who cannot write civil posts and fails to show any consideration for others' feelings.