I think sites like Reddit emerged at a point where infrastructure for sharing was very valuable for users in its own right. It seems amazing now that the submit/upvote/downvote flow was so novel and we all lapped up the spirit of freedom. But actually the powerful feature of sites like Reddit and HN is the moderation rather than the technology. I would pay for some of the better subs, and certainly HN.
Good moderation is invaluable and it's a shame that it is hard to make money out of. I have a hunch that Google Contributor is aimed at this market as a sort of Netflix for content.