I think you're spot-on. This has to be a part of it. Whatever its issues, the visible point system was a way for readers to interact with the site in a way that

I think you're spot-on. This has to be a part of it. Whatever its issues, the visible point system was a way for readers to interact with the site in a way that produced a response for all to see. Whether this encouraged herd behavior or changed the shape of discussions in an unfavorable way is another issue. Comment-dimming notwithstanding, that feedback has been removed. There's a class of reader whose desire for a kind of interactivity is no longer satisfied by the voting system; for them, pile-on voting has been replaced by pile-on posting (albeit often in the more thoughtful HN style). Which in turn begets more such posts, and thus a more polarized conversational shape.