I still go back to saying the most important thing that you've ever written, for me personally, was CFAI section 2.2, where you argued against anthropomorphism in AI. That was the thing that cracked my mind loose and set it on its journey without feeling trapped by traditional views of rationality, intelligence, or wisdom. Not that this appeared on OB as far as I can remember, but I think it's important enough that it should appear somewhere in the book.
Followup to: The Most Frequently Useful Thing
What's the number one thing that goes into a book on rationality, which would make you buy a copy of that book for a friend? We can, of course, talk about all the ways that the rationality of the Distant World At Large needs to be improved. But in this case - I think the more useful data might be the Near question, "With respect to the people I actually know, what do I want to see in that book, so that I can give the book to them to explain it?"
(And again, please think of your own answer-component before reading others' comments.)