> One setup I could see working is a pubkey setup where anyone who posts, always encrypts with their private key, and anyone you want to hear from, you add their public key to your list of keys you decrypt to read.
The most important thing that web forums like this one enable is interaction between people who don't know each other.
People could auto-trust those who have been trusted by those they trust; this could have a user setting deciding how many degrees of separation they are willing to trust. Everybody's public key (ie. the ability to read their signed posts) is public, it's a question of which ones you choose to read.
There are many ways to skin this cat, but every one that's gonna work involves cooperation, rather than adversarial control.
The most important thing that web forums like this one enable is interaction between people who don't know each other.