When disagreeing, please reply to the argument instead of calling names. "That is idiotic; 1 + 1 is 2, not 3" can be shortened to "1 + 1 is 2, not 3."
Don't be curmudgeonly. Thoughtful criticism is fine, but please don't be rigidly or generically negative.
Please don't fulminate. Please don't sneer, including at the rest of the community.
Please respond to the strongest plausible interpretation of what someone says, not a weaker one that's easier to criticize. Assume good faith.
Eschew flamebait. Avoid generic tangents. Omit internet tropes.
Please don't post shallow dismissals, especially of other people's work. A good critical comment teaches us something.