[msarnoff]

The language is one thing, but the Objective-C runtime is brilliant. Read about the implementations of key-value-coding, key-value-observing, and bindings to get a sense of the bonkers things that have been done with it.

And ARC (automatic reference counting) blew my mind when it was first released. Rust was a blip on the radar, so the idea of automatic memory management, handled at compile time, without GC, was amazing.