This one didn't surprise me as much as I hoped that it would. I've probably been reading too much fanfiction, but I can't help but think that Rowling gave a few too many clues out about OotP for me to be completely surprised. However, she redeemed herself with the character of Umbridge, who is so horribly awful that she is probably one of the best (and I mean that in the horrible, most annoying ways) characters in the series. She's one of the few characters I've ever truly, truly hated in all of my reading.
This book also has one of the best chapters ever written, called "Career Advice," and if you read it, you'll see why. I clapped, I really did.