Phrasing it another way, it is choosing at what level to talk to the reader. On the one hand, I really want the reader to be able to make immediate use of the concepts that I am talking about, which drive me to do more practical demonstrations, code samples and covering more common situations. On the other hand, those take up a lot of room, and they tend to be boring if you don't need exactly what you need right this moment.
High level concepts, open ended possibilities and assuming a bit about the reader knowledge level makes for a book that is much more narrowly focused, and I think that it more valuable. However, it also tend to leave readers unsatisfied, because not everything is explained.
Currently I am writing a UI focused chapter, and to get a good experience from the UI you need to invest a lot of time. Metric tons of it. I am trying to chart the way and show how this can be done, but without getting mired in all the actual minute details.
This is a tough balancing act, and I am not sure if I am succeeding.