1. Be respectful.
2. Stay on topic.
3. Move the conversation forward. This sounds like a repetition of #2 but there is often a distinction in that, say, a discussion about a new product feature is likely not the time to discuss the company’s history with features. Going in that direction is moving the conversation sideways.
4. Provide supporting evidence for what is said. Claiming something like, “I’d never buy this from Company X” is a baseless statement compared to “I’d never buy this from Company X because A, B, and C are an indication I won’t get much support beyond the 90-day warranty and that’s not enough at that price point.” The trick I use for this is to include a word like because since it compels an explanation.
5. Avoid attempts at humor. For one, text mediums like HN can easily lead to misinterpretations; there are many people reading for whom English is a second language, so being clever can cause confusion for those readers; if my humor were so good to be worthwhile for the amount of readers a place like HN has then I should be a comedian. I'm not a comedian.
The guidelines may say that humor should be avoided, but the readership (the people who, in the end, decide what HN is) seems to disagree.
I don't think humor in general is a bad thing (my friends would likely in part describe me as a wise-cracking, sarcastic jokester at times), but I just don't think HN -- or any sort of textual medium where participants don't know each other that well or at all -- is a great place for it.
Obviously we're all free to disagree on this (and we obviously are, given the size of this subthread), but I think overall the community agrees (through up/downvoting) that humor on HN should be fairly rare.