I can't say much about textbooks from the perspective of the producer, so I'll say something from the perspective of a consumer. 

I like books that are good at what Terry Tao referred to as "information hiding".

http://terrytao.wordpress.com/advice-on-writing-papers/create-lemmas/
http://en.wikipedia.org/wiki/Information_hiding

My favorite textbooks somehow manage to give me all the excruciating details when I need them but only the intuitive picture when I don't.