While true, it is also not a given that you'd need to trust _all_ of HN. I visit sufficiently regularly that I see some people post and recognize their username. I often think -- I wish I could _follow_ them. Not so much of a stretch to think of rings of trust built around particular users. Bringing people in and kicking them out of these trust circles could play a role. PageRank -> TrustRank? Of course it would also be only one metric, among many possible trust rankings and many possible other signals and settings.

I bet the (niche) product as such wouldn't be as hard to build as it would be to scale. Imagine every user constantly tweaking (directly or indirectly) their search result settings, and having that impact millions (or more) indexed items, for every user.