I found this novel to be the most difficult of the Harry Potter stories to read to date, and the reason was undoubtedly the character of Dolores Umbridge. She is the embodiment of petty authoritarianism backed by government. Though her powers do not begin to approach the evil of the major villains of this universe, her low-key cruelties are somehow more painful. A large part of that is to do with the school setting, where (even with their magical powers, as developed by Dumbledore's Army!) the students are terribly vulnerable. It is fitting that Umbridge is ultimately destroyed (or at least pushed out of power) by elements - the centaurs - which are anarchistic, simply not recognizing the authority that backs her.
This novel is also about government authority created and held by the control of information. Rowling clearly depicts an official, state-controlled news medium, and the necessity for the alternative press, however wacky. The crucial event at the end of this novel is not an actual defeat of Voldemort's forces (in fact, an important character dies, a defeat for the good side), but is a breakthrough in the wall of "fake news" which denies Voldemort's return, and thus enables it. Voldemort is also thwarted in his quest for a vital piece of information (the prophecy about himself and a child), while Dumbledore significantly strengthens Harry's position by giving him a great deal more information at the end - information, he regretfully informs us, he wrongfully withheld just out of sheer affection for the boy.
The novel reaches a highly cinematic conclusion in the labyrinths of the Ministry of Magic - were the movies in view or even in production by the time this was published? - which manages to be imaginative, gruesome, and occasionally deliberately funny. In the midst of that, we also get to see for the first time a direct Voldemort-Dumbledore battle, in which Voldemort takes advantage of the mysterious synergy between Harry and himself but fails to make Dumbledore destroy Harry as a side-effect of attacking Voldemort. This part of the story also reinforces the elevation of Neville Longbottom from a comic foil to a major character.
My goodness, that's a very thematic analysis. Corking good story as usual, but because of Umbridge, I was rather glad to leave it behind and move on.