It's "the programmers way" to try to come up with some sort of moderation/policing system to solve this. Please do not come up with a moderation system.
There should simply be a confirmation step after hitting submit, reminding the poster that they are valued for their positive contribution and giving the poster the chance to rethink their post and cancel if its too negative. Additionally when posting, require people to self-rate the level of negativity of their post when they submit from one to ten. Publish that number along with the post.
There's nothing at the moment that explicitly asks or reminds people of expectations when they post. What is it that makes you think posters know there is an expectation not to be negative? People don't read the community guidelines every time they post, UNLESS key points of the guidelines are presented to them.
The community moderation system, what it ends up being, will be gamed and abused and alienate and anger people. Don't do it. Witness the toxic mess of StackOverflow.
HN is not in a position right now to express concern about negative posts because it is not explicitly asking people to avoid negative posting, at the time that they post, and requiring the user to think about it. That is step one.
HN would be implementing a passive-aggressive solution by going to an indirect system that continues to allow people to post negatively and then gets the community to police it later. Address it up-front, at the time of posting. Let people be responsible for their own posts. Asked to think it through, posters will dial back the negativity.