I think the explanation of how and why quantum mechanics is perfectly understandable and explainable would be the thing most likely to make me give the book to someone else. In many ways, its also the most valuable thing that I've taken away from the Overcoming Bias site.
I started out in Physics, and while in College and University, I often encountered the "don't try to understand it -- just do the math" point-of-view and I never subscribed to it. I always felt that there should be a way to make it all make sense, but I was never capable of doing so myself.
The articles on QM from Overcoming Bias resolved many of my doubts about how things worked in this realm, while showing that everything could be described by a consistent model that actually made sense. Its hard to overstate how happy I was to see that.
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.)