Slight SPOILER ALERT coming up - Be forewarned, ye who enter here
Here, halfway through the seven books, the mood shifts perceptibly.
It happens long before Harry actually encounters Lord Voldemort in the flesh - although that is a pivotal moment to the whole series - and it makes for a darker, more demanding story than the imaginative children's enjoyment that constitutes the previous three books.
This book is also a lot thicker; generally a sign that the creator has fallen in love with his/her characters, which is not always profitable to the opus in question.
In this particular case, however, I happen to be just as much in love with her characters myself, and so the one drawback I can see to this books' 800-page length is that it's too bulky to be conveniently carried around.