This is probably my bias as an engineer showing, but the technology doesn't seem like the hard part-
I always understood that having an resilient network means people will use it to post some bad things, but I don't know if I really internalized the scope of that until later.
I had originally envisioned it might be useful in oppressive countries, where people needed a way to communicate - Recent events have shown how dangerous that can feel when you're in the midst of people who feel like that describes them.
As another HN post pointed out, there are two natural audiences for such networks - Idealists, and those who can't get away with stuff on other networks.. And the second is going to be far more common. That will influence the culture, and help to drive other "good" people away from the service, amplifying the effect.
Even if you have user-selectable moderators (Which I had, similar to the request the author makes), without a huge war-chest to hire a large team of default moderators, you'll never be able to keep up. The default experience for the average user will be terrible.
Over and over, I ran into issues like that - It's relatively easy to built the technological network, but managing the social network aspect is an unsolved problem.