Do others have examples of software "where the way it works is a crucial part of the user experience"? The one that comes to my mind is lisp macros; if you mess with them for a month or two you can't help but have a pretty good understanding of how they work. Clean internals can colonize your brain in a way that merely clean interfaces can never compete with.
I also think this ties in with the law of leaky abstractions, in that a good understanding of that law will make ui designers choose lesser/thinner abstractions over bigger/leaky ones.
