You can get a glimpse of why her books have impressed so many -- or at least a glimpse into her mind and thought processes -- in my interview with Betsy, which you can find here:
http://www.chronicles-network.com/fo...tsy-james.html
I tried to coax her into saying more about YA literature in general, but it became clear that she writes what she writes without thinking too much about what age group it's intended for -- just tells the story that she needs to tell, in the way it needs to be told.
She had some very interesting things to say, however, and she did give a long list of favorite books for younger readers. (If any parents here are looking for recommendations.)