One would be surprised at the evolution of Rowling's writing. The story gets some depth, and it's not as naive as the first two books; I am, however, disappointed to see that the most complex character of the series, prof. Snape, is still a caricature more than anything else. It's obvious that by this time, JKR didn't plan for the end very well, therefore the lack of depth in some important characters. It is interesting, however, to see the build-up of romantic relationships between the kids; unlike the Snape character, this seemed to be well thought in advance.

This book marks the beginning of the departure from the naive perspective of the first two books. There are consequences, there are tangible evils like the dementors. Some of the details left out from the previous books are explained, including the imprisonment of Hagrid.

I think this is a first step from children's books to young adult fantastic story. The change in tone is timid, and breakouts of naivety will baffle the young adult while the outburst of depth may be unnoticeable for children. But this is no longer a children's book, just a naive young adult story.