The fight against Lord Voldemort is complicated when the Ministry of Magic - suspecting Dumbledore of making up Harry's story of Voldemort's return in an attempt to take over the Ministry for himself - gets involved. And, of course, another year means another Defense Against the Dark Arts teacher, and this one is the worst yet! 
 One thing that has really impressed me about the Harry Potter series is that they seem to grow with their readers. The first few books were geared towards children. Now that Harry is in his mid-teens, the plots are becoming more complex, the stakes are higher, and the issues tackled are more difficult. 
 In Order of the Phoenix, we finally drop the idea that the world is divided neatly into those who are good and Deatheaters. We knew who the baddies were and, whenever Harry didn't like someone (such as Draco Malfoy), it often turned out that s/he was in some way associated with Baddy-in-Chief Voldemort. But now, we encounter the Ministry - a group that is misguided and often takes a stand on the wrong side of the issues, but isn't 'Capital E' Evil. 
 Rowling did some very interesting things with the theme of identity in Order of the Phoenix. In addressing the issue of bullying - which has previously been covered with Harry as the victim and Draco Malfoy as the bully - she now shows us Harry's father (with whom Harry identifies) as the bully and Professor Snape (with whom Draco is frequently paired) as the victim. Harry is therefore forced to confront, at least in hypothetical form, the idea of himself as a bully, and to take a stand against possible self. 
 Rowling further challenges Harry by putting him inside Voldemort's head, making him become Voldemort, so that he must actively choose to define himself as a goody. 
 Because the books are from Harry's perspective, we often see his friendships as very one-sided; Ron and Hermione exist to support him, Dumbledore exists to mentor him, Professor Snape exists to challenge him, etc. Hermione has always been something of an exception as we see hints of her friendships outside of the Harry-Ron-Hermione circle - friendships with other girls and, more recently, her relationship with Viktor. In this volume, Ron starts to find a separate self as well in his love for Quidditch. It was great to finally see him grow a little. 
 I love the way that Rowling captures the reality of being a teenager. Harry's exchange with Professor Umbrage over learning practical skills versus learning just enough to pass a test could have been lifted almost word for word from my own classroom experiences. His nervousness and awkwardness as he attempts to start a relationship with Cho Chang was far more familiar than I would have liked as well. 
 The last thing I wanted to comment on is Rowling's skill as an author. I complained (a lot) during the first three books about how contrived her mysteries were. The plot moves forward, seems to get stuck, and then an utterly ridiculous overheard conversation or found item allows it to move forward again. She seems to have largely grown out of this by the time she came to write Order of the Phoenix. The plot construction has now matured enough to have largely caught up with the exciting world and fascinating characters.