Feeds/timelines are first-class citizens in the AT protocol and are decoupled from account hosting.

On Mastodon, your timelines are computed by the same server that hosts your data. Consequently, signing up to a server to have an account on the fediverse is the same thing as joining a community. You follow the servers rules and share the same local timeline as everyone else on that server.

On Bluesky, feeds are arbitrary, fungible and provided by any server, and it can be computed/curated/moderated however they like. So communities are “built” around feeds rather than around account hosting providers.

The AT protocol also has “real” account portability (though I have not seen this demonstrated in practice atproto.com/guides/overview#account-portability). On Mastodon, account “portability” is a delicate dance that requires the cooperation of both the origin and destination server.

Mastodon has something that Bluesky currently doesn’t: real federation. The Bluesky server that everyone signs up to doesn’t federate with anyone else, since the whole protocol is still a work-in-progress.