JKR did a great job showing that evil doesn't always have to be snarling, pure evil like Voldemort, but can also be someone who is so sure of their righteousness that they become evil, like Umbridge.
Like Hamlet, there's a bit of a play within a play for me here, too. The situation in Hogwarts represents what's happening in the Wizarding world at large. It's so beautifully written and subtly done.
Umbridge just glows with her self-righteousness and she's my favourite of the minor villains. She is truly loathsome.
This books introduces a minor character who became a more major character: Luna. She was the only non-Gryffindor to fight with them at the Ministry. I wonder how she would have been as a character if she'd been around since the second book. I absolutely love Luna.
The DA is so important, not only for its resistance to Umbridge, but because it shows that Harry is a strong wizard in his own right. Until OotP, people had been telling me that Harry wasn't much of a wizard, and always relied on his friends. I never understood it myself, but it was officially silenced after OotP.
We see more of the trio growing more mature. Harry becomes very angsty, very teen-like, sure that his problems are the most important and that his views need to be heard. His stubborness in not accepting that his visions could be counter-intelligence from Voldy-baby is pure Harry. He's more clearly shown here as impulsive, but with his heart in the right place.
Hermione/Ron shippers became much more sure about what they thought here. Ginny/Harry also became more clear, even as Harry crushed on Cho.
The ending scene with Luna talking to Harry about Sirius is one of my favourite scenes in the books and the genesis of my Harry/Luna shipping.
