You mention the filter effect: I think what really killed Usenet, at least for me, was spam, much of which was automated, with no great mechanism in place to combat it.
At the same time, paying ten cents to make a comment on Reddit or HN is very unappealing.
1) Start off with a trusted group of people (stakeholders)
2) Set up an invite-only system
3) Inviting someone means sharing some stake (i.e. some reputation) with them
4) To post you need some minimum amount of stake
5) With every post that's not downvoted into oblivion you increase your stake until you reach a certain equilibrium point. (So there's a limit and it's not about gaining status points.)
6) Posts that are downvoted into oblivion will cause their authors to lose some stake.
HashCash is older than Bitcoin, and Bitcoin sybil-attack protection is basically HashCash.