that upvotes are visible to the user is a big one. Hackernews (or reddit for that matter, which employs a similar mechanism), would work just as well for the purpose of conversation if your score was hidden.

But seeing that number next to your name climb up is something that drives a lot of engagement on the platform. I don't think this is intentional on HN's part, but on Reddit for example I have no doubt that the activity generated by the 'karma' mechanism is left in place precisely because it generates so much (meaningless) content.