"Be civil. Don't say things you wouldn't say in a face to face conversation. "

It's a pretty good rule for writing things on line whether it's in the guidelines or not. Sometimes you (meaning: all of us) get tempted to be snide or snarky, but if you think that there's another person out there, and if you pretend you're saying whatever you have to say to them directly, I find that it greatly improves what you write.

It doesn't mean "don't criticize", just do it as if that person were in front of you.