Ask HN: Favorite aspects of Cocoa/NeXTSTEP for app dev?

What's your favorite parts of Cocoa? Personally I've been a Mac OS X fan for a long time. Before that Étoilé was a favorite.

Is there anything in the programming side or design of NextSTEP and later Cocoa that lent itself to so many beautiful Mac OS X apps being made?

I've been working on an implementation of OpenStep and Cocoa. The first piece was dynamic runtime methods and protocols. It made adding a view inspector trivial!

However were dynamic/runtime methods the only magic of Cocoa? It feels like we've lost some of the art of desktop apps.