I'm not going to do the protocol justice in an HN comment, but a core goal is to separate the entities that control the data, the entities that run the network, and the entities that control the interfaces (and recommendation algos etc).

If your Mastodon admin goes rogue, you a) have to get your data out of their hands, and b) have to rebuild your Mastodon presence in a new location, all your connections, history, etc are gone. That's exactly the same problem that Twitter users faced.

On ATProto, you can self-host your data while still making use of Bluesky/etc, in a way that you can't do with Mastodon or even email. You can choose who your UI comes from and just switch UI and keep everything, you can switch who your moderators are and keep everything else, etc.

Check out Blacksky, they're doing amazing work running a whole separate ATProto system that is fully compatible with, and actually the same social network as Bluesky. You can transparently migrate, at a protocol level, something Mastodon/ActivityPub doesn't attempt to tackle.