More realistically, for all the places where a web-of-trust model fails moderation seems like the perfect fit for it. I see no reason we can't have multiple teams of moderators whitelisting/blacklisting different content in the same namespace, with their changes applied only to users who opt-in.
Federated accounts between servers might be a nice overlay feature set. Servers can just reject mail from any unauthenticated user, and sync user databases periodically.
Federated moderation enables extremists to blacklist people who endorse something they don't personally like and limit their ability to use other sites.
I have similar thoughts. Perhaps solving that problem would still count.