I don't need to say Spoilers Ahead, but I will anyhow...
That's good enough.
Getting this out of the way first, the bad, IMO:
Harry in the first 1/3 of his after-muggle scenes in the book. Too tempermental... It just really seemed out of character for him.
The thing with Cho... Ok, sure, you wanna give Harry a fleeting love interest. Fine. WHY CHO? She really doesn't seem like all that great of a prospect, constantly throwing little fits at Harry whenever he mentions Herm... I mean, anyone who had payed attention at Hogwards *knew* they were long-time friends, so why is it a surprise that he still is? (Of course, I have little luck on the romantic front myself, so this might be part of the problem... Expecting common sense and all.

)
The thing with Percy... Either that better get resolved before the end of the series, or we better get an explanation better than "Percy wants to get promoted" for Percy acting, like, well, to quote (or nearly so) Ron, "the biggest git in the world".
Death of Sirrus. Bah. I liked Sirrus. Sirrus was cool. And his death didn't even have meaning.
The good:
The book in general read very well.
Is it just me, or did this one seem to be less regionalized than they have done with past books? There were a lot more british-isms I noticed in this one.
Developing Neville's character.
Luna (Even if they didn't do much with her)
The "Harry Potter Realed" interview, and the after effects of it.
Seeing the reactions of the entire school to the new rules as they got introduced.
McGonagall. She kicked ass.
Umbridge. Evil as hell or not, she was a great character. Great character. I loved her slipping more and more from the "Hard-hearted semi-evil" Umbridge to the maniac, out-of-control, paranoid, semi-inane, pure evil Umbridge at the end. And I LOVED her fate, and I LOVE that Rowling left it very vauge what happened to her. My mind has invented a number of decidedly Un-Eric's Grandma Friendly (TM) ideas as to her fate... Considering her reaction to the sound of hooves...
Fred and George. Fred and George! Awsome characters! Just awsome. Their exit was amazing, and it showed so much character that they were willing to do what they did... Class clowns or no, they knew they were risking a lot doing that, not just being expelled (Which they didn't care about, of course).
Dumbledor's exit when DA is revealed. Like the painting said... "He has style.".
Dumbledor at the end.
The D.A.
The portrait of Sirrus' mom. I have relatives that would do that if they could.
The scene where they tried to take Hagrid, and he went berzerk... Even if I most assuredly did not like what happend to McGonagall during that.
Things I'm still trying to figure out my feelings on:
The memory of James Potter as a student. Um... Not how I imagined him. Sirrus' explanation doesn't really mollify that, either.
Hagrid. He just seemed... Off his game this book. Even for him.
Occlumency... It doesn't seem like a magical art so much as "Let me torture you mentaly and you develop a resistance to it.". But, then, we never really saw much of it, so...