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.
Please don't use Hacker News for political or ideological battle. That tramples curiosity.
Please don't pick the most provocative thing in an article or post to complain about in the thread. Find something interesting to respond to instead.