> If some instance of a federated system is allowing child porn or other criminal content to flourish then they should be shut down. Every instance would need to be held to a baseline standard for dealing with such content.
Yes, moderation. The ideal isn't a system that is un-moderate-able (for the reasons listed, among others), its a system where individuals can choose their moderators based on the differing goals and preferences of the online communities (but within some reasonable bounds/process for limiting rights-violating content like child porn, criminal incitement, etc)
... so moderation, then?