Before I begin, I'd like to point out, no, these aren't rules. I can't make them rules, and I don't plan on harassing you all with them continuously. I am, however, sick of seeing general rudeness on the comments section, and I figure having some system of etiquette can't hurt, especially if it's voluntarily adopted. I personally feel that this site, for all of it's positive development over the last 2 years, could be even better if we treated each other with respect and dignity when commenting.
1. Don't make a comment on an entry editor's worth as a human when commenting, or harass them at any level. This is probably my biggest pet peeve when it comes to comments. Yes, people make entries that aren't legitimate. Yes, people make entries that are only attempts to spread their own content. This does NOT make them worthless sacks of flesh. I have too often seen comments that equate to "Go commit suicide for this stupid entry". In any other context, this is hate speech. Any kind of harassment for an entry, such as "get off the internet", "OP is brainless", or anything along those lines, is not kosher.
2. Don't downvote for an opinion you don't agree with if it's well-thought out and calmly presented. There are indeed trolls and ragers who comment, and you can downvote them all you want; that's what the system is for. But there are going to be people who you disagree with, and it doesn't help matters when we neg them into oblivion.
3. Don't start flamewars in the comments. It ruins the opportunity for constructive comments, as everyone is going to want to jump in. If you've got an issue with a commenter, take it to them personally (though you should be respectful on their wall as well).
4. Don't troll. I think it's worth making a note about this, even if it's an unspoken rule already. The comments are a section for constructive criticism or discussion on an entry, and trolling just harms that purpose.