They're actually more similar than I expected. Nostr seems to have gotten lost in trying to be a decentralised store of everything [1], while BlueSky is focusing on Twitter-style social media and its challenges—moderation, portability, etc. (though I'm unsure if ATproto can support other types of decentralised content)
Nothing wrong with being a store of everything like Nostr, it's just that a totally open model of contribution with no direction or oversight (well, except fiatjaf) tends to end up bogged down following a million different and often competing ideas. It's very active, but moving in 196 directions: https://github.com/nostr-protocol/nips/pulls
Once you, the developer, are required to implement (thiskey === thatkey) a relay operator has to index the entire network before a post can be rendered in compliance with that NIP. It does bog it way down, as you mentioned.
this public key does not equal that public key either as much as we might wish they could.
