Simple things, like actually spelling out the steps of a derivation vs. "Here's equation a, from which we trivially derive equation b" but it might take you a while to figure out the steps if you haven't done a lot of Calculus problems recently.
And Mitchell just seems really good at explaining things (having heard him speak in person a few times).
http://www.cs.cmu.edu/~tom/mlbook.html
Simple things, like actually spelling out the steps of a derivation vs. "Here's equation a, from which we trivially derive equation b" but it might take you a while to figure out the steps if you haven't done a lot of Calculus problems recently.
And Mitchell just seems really good at explaining things (having heard him speak in person a few times).