I like the simple explanation of K-Means, and I like the contrast with the dense set-theoretic language -- a prime example of "mathematosis" as W.V.O. Quine put it.
I admit that was an unfair swipe at formalism, but I do think that formalism is far more effective in the light of a clear, intuitive exposition that conveys the concept straight into the mind. Formalism is good for battening things down tightly, after the fact.