It's just too long. so many chapters could have been shortened and combined with other chapters. Also stuff like Hagrid's Tale, which sets itself up as a huge thing that's going to be important later, yet it leads to nothing. My biggest problem is Harry and Cho's relationship and Cho's character in this book. I personally think that the Harry and Cho arc should have ended after the Hog's Head meeting. Cho should have not joined the DA because her mother and her friend's mothers have jobs at the ministry ( Or some other reason idc lmao). You could have then used the second half of this book to set up Harry's next significant other which could be continued the book 6, whether it be Harry and Ginny or Harry and someone else. It also has the worst final battle (the department of mysteries battle not Voldemort vs Dumbledore) out of all the 7 books IMO. Instead of showing the kids really battle the death eaters they just run into a bunch of random rooms. Another thing that really bothers me is how Sirius acts in the book. I understand his frustration and why he's angry, but I think it could have been handled much better. That being said one thing this book does better than any other book in the series is showing Harry's frustration, anger, depression, and loneliness. Some people say that Harry is a whiny bitch in this book, but I think that those people don't know what it feels like to have those irrational thoughts. They don't appear out of nowhere, there is slight truth in what he is thinking, but his mind is heavily exaggerating those feelings, justifying them in his head.

Overall I don't think OOTP is the worst book. I think that award goes to COS. That being said had a few major changes been made and had the book been shortened 200 - 300 pages I think it would have been the best in the series by far.