Adversarial migration: The ability to migrate servers even if the previous server is down; (1) the ability to prove your identity (2) the ability to broadcast the new identity to your followers (3) the ability to keep a continuous backup of your content and (4) the ability to import said backup into the new server.
I think this is vital for users joining a non-friend server (safety in friendship), and a non-corporate server (safety in numbers and professionalism). I think right now we are in a bad middle ground (new users joining servers not run by friends nor corporations) where admins have the final say, yet users cannot migrate easily, especially if they value their old posts. I found that this is a big emotional put-off; I fear joining third-party PixelFed servers because it is hard to migrate and would prefer to self-host, but not everyone can do this.
Anecdote
A male nudist I follow had an account on mastodon.lol, completely CW’d except the profile picture where it is top-nude. Got told not to do that (the mods wanted to apply the bare-chested policy to both genders). Grudgingly obliged, but got banned for critisising the rule. Couldn’t even migrate; I told him to ask the admin to temporarily unban him to migrate but they didn’t even let him do that. I think the nudist-verse is lucky that he didn’t just give up and took a lot of effort to set up everything again on a nudist server. This is why I think a adversarial migration functionality is vital if we want people to have more trust in non-corporation-run servers – at least you won’t get banned for critisising moderation rules there 
A good compromise for a Fediverse without the ability to easily switch servers (if you value your old posts), I think, would be people-run servers, perhaps by democracy. I think social.coop is one of them. I think qoto.org is another one, too. Unfortunately it is very rare.
But I think the better alternative would still be to make servers easily self-hostable by the average person, or to have adversarial migration (which would still be useful in friend-run servers, but perhaps not as much).
Here’s a related article by a server implementing “HO-LI moderation”, intended to reduce centralisation and “safe large instances”: https://fedimedia.packmates.org/custom_pages/holimoderation.html