Clarity and transparency. One should be able to open the book to a page, read an argument, and see that it is right.
(Obviously this trades off against other values -- and is in some measure a deception --, but it's the kind of thing that impresses my friends.)
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.)