The biggest different is identity portability. In mastodon your identity is tied to the server you registered on. If it wasn’t your own server you don’t own your identity. On nostr, your identity is your keys and thus can migrate relay, client & NIP-05 providers at will.
Discussion
Good point! And most users just pick a server at random or choose one they know someone else is on, without ever knowing who runs it or what their reputation is. And then if you want to migrate later, you have to rebuild your identity at another server and more or less start over.
Fair point. In general I like that nostr can power different kinds of applications. Not just another social network, which is of course the way it is being reported, instead of a protocol.