Be kind. Don't be snarky. Converse curiously; don't cross-examine. Edit out swipes.
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."
Please don't fulminate. Please don't sneer...
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. It tramples curiosity.