It’s the book that transitions the series from mostly mild-mannered to very, very ominous.
It never feels slow to me (though I’ve heard the opposite from other people. Ahh well, different strokes, different folks).
It introduces a side to Harry that startled a lot of readers but made me fall in love with him even more. I think Angry!Harry is very important and rounds him out as a character. Before he seemed sort of super human with how much he could bear before it got to him. I also think his anger was mostly justified so I don’t begrudge him his outbursts.
I love what the DA did for secondary characters like Neville and Luna. Neville’s confidence grew during those lessons, and Luna didn’t feel so alone. As much as I love the Trio, it’s so nice to get to read about someone other than them sometimes, and OotP fills that need.
We see the Wizarding World broaden in this book. Before, we knew places like the Ministry and St. Mungo’s existed, but OotP actually takes us there.
The Order. The Order The Order The Order.
Umbridge’s regime did a great job showing just how united staff and students could become when faced with something evil, which is pretty important.
Snape’s Worst Memory. Any peek into that era I can get, I’ll happily take it and cherish it forever.
The Battle of the Department of Mysteries, especially Dumbledore facing off with Voldemort. So rad.
"By all means, continue destroying my possessions. I daresay I have too many."
THE TWINS FLIGHT FROM HOGWARTS.
It’s not just my favourite HP book, it’s my favourite book of all-time and the one I’ve reread the most. It also holds the special memory of being the first HP book I attended a book release for.