Donwvotes on HN are anonymous and silent. This equates to being slapped in the face by someone with a mask on while you're in the middle of a conversation, without knowing why, and not being able to do anything about it. In a real conversation, disagreeing with someone involves actually opening your mouth and talking to that person, and even then is not rewarded with an immediate punishment to the person you're disagreeing with. This is what leads to a discussion, or better yet, a debate. Instead, the only recourse on HN is to change how you talk or to keep quiet.
But at this point I must assume this is an intentional design decision, and I adjust my tone and substance accordingly. But I wouldn't be writing this if I weren't okay with HN's rules.
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(edit/addendum: this isn't to say there are no discussions or debates on HN... but I hope other's agree there is something uniquely HN about most of them)