Whereas in ATproto you have to find a place to register an account, say a domain for yourself or use someone else's domain, in NOSTR creating an account is nothing more than a public/private key that is generated on the fly.
Then is the storage of information. Whereas in ATproto information is stored in plenty of different formats and specific folder configurations which are easy to miss, in NOSTR is all JSON from a single base template file.
Third is the absence of global moderation and islands. Whereas in ATproto only a few (I think 3 main providers) exist, you can be kicked out from them and effectively stop reaching your audience. In NOSTR if a relay stops receiving your texts it is a bummer but there are hundreds of other volunteer-based servers to where you can send your texts, so it is very difficult to block people from reading texts of the authors they are interested.
Simplicity is the key. NOSTR is the first and only network that I've seen so far where content/authors are truly disconnected from the servers hosting their data.