Conceptually, Mastodon is a bunch of copies of the same webapp emailing each other. There is no realtime global aggregation across the network so it can only offer a fragmented user experience. While some people might like it, it can't directly compete with closed social products because it doesn't have a full view of the network like they do.

The goal of atproto is enable real competition with closed social products for a broader set of products (e.g. Tangled is like GitHub on atproto, Leaflet is like Medium on atproto, and so on). Because it enables global aggregation, every atproto app has a consistent state of the world. There's no notion of "being on a different instance" and only seeing half the replies, or half the like counts, or other fragmentation artifacts as you have in Mastodon.

I don't think they're really comparable in scope, ambition, or performance characteristics.

— Dan Abramov, Hacker News comment discussing his Open Social article