Intuition was much more than just the
Amiga's
windowing system. It was a beautifully
designed architecture for developing user interfaces.
Intuition used the Amiga's amazingly sweet message passing kernel "exec" as the basis for its message passing/event oriented needs by extending the basic execsystem with intuition specific messages for events like window open, window close, mouse move etc.
Intuition was largely designed by R. J. Mical whose previous claim to fame was the righteously great arcade game Sinistar. His great sense of humor pervades the design with API calls like GimmeZeroZero() etc.
Writing code for Intuition was a dream. Everything made sense and was well documented in the Amiga's Rom Kernel Manual or RKM.
Ah, for the good old days :)