I couldn't love this book more. The intro paragraph is right. THIS is where Harry Potter gets interesting.
The plot is largely focused on the characters and their relationships with each other. Why does Snape hate Harry so? Did Sirius love his Potterites or was he a spy? Can Lupin bare to bond with Harry? Harry's happiest memory is a fantasy
of his parents? Oh, good, the Woobie potential. And Sirius and Lupin's reaction to the ending mystery was heartbreaking.
The magic intrigues and invokes emotions: scary at times (the Dementor), hopeful at others (the Patronus), hilarious at others (the Boggart). Yet they tie into the same theme: shit is scary and life has terrible monsters just looking for ways to suck the life out of you, but you can never - give - up. Plus the magic isn't background fodder or tools to be used at human disposal. It is living, wild, threatening and temperamental and the value of a wizard is his ability to understand and work with that magic, not try to overpower it: Buckbeak and Malfoy, anyone?
And the plot... I know, I know, time travel rarely seems like a good plot device to use, but I liked this one. It was a stable time loop where the future events already happened and Harry just never saw it. Great! It used a magic that was already present throughout the story, making it something understandable upon the revelation (plus, not a DXM). Hermione was in two places at once, so was Harry. I loved how EVERYTHING fit into the climax. Sirius, Snape, Lupin, Scabbers, Buckbeak, Dementors and the Timeturner/Hermione's lessons.
Everything fit into a nice bow that answered many questions and raised new ones. "Will Harry get to leave the Dursleys now?! Oh crap, Wormtail's going to go to Voldemort?! Noes!"
And, as far as series development, it gave Harry several new allies, taught him more than one spell, gave him diverse enemies to fight and deepened Hogwarts as a setting (plus, introducing Hogsmeade) and the Ministry of Magic as an institution. Which is more than what I can say for some bulkier sequels...
This is probably the only Harry Potter book I will ever own. Plus, it's only
435 pages (US).