It was also fairly difficult -- the assignments were hard, but at every step, you could look at what you'd done and say "I know why I'm doing this, and I can see how this works."
I remember at the end of the term he took several students' notes and made copies of them, so that he could compare the students' notes with what he was trying to convey, and could know if he wasn't teaching certain parts of the class well enough.
It's a shame that not all professors are as dedicated and responsive to their teaching obligations as Prof. Abu-Mostafa.
Oh, also, "introductory" in this context is meant to differentiate it from "graduate level". Every student (mostly juniors and seniors) in this class will have had several terms of math, theoretical CS, and practical programming classes.