I am rereading the Harry Potter books one by one at the moment, and have finished The Order of the Phoenix yesterday. Overall, it was pretty great, but there were two things that annoyed me:
First of all, the book is huge. It contains almost 800 pages, and at page 250 (at that page, The Philosophers' Stone was already over) the story was at the Friday of Harry's first week at Hogwarts. Now, I don't mind the length of the story (Hey, more of it to enjoy), but the physical size of it. You have to read the book on a table or some other form of support, as your hands quickly tire from holding it. In fact, I wouldn't be surprised if you could beat someone to death with the hardcover edition (You can take it on an aeroplane, though).
Second, and more importantly, why was Harry being such a complete idiot at the end? When he and the DA rushed off to the Ministry of Magic to save Sirius Black, I mean. I counted no less than seven things Harry could have done, six of which would have prevented the battle in the Department of Mysteries and thus Sirius' death, while the seventh would have just prepared them better:
1: Harry could have just looked in the mirror Sirius had given him for Christmas to check if Sirius was at Grimmauld Place. Yeah, he had forgotten about Sirius' present and not unwrapped it, but how likely is that? He wanted to talk to Sirius all the time, and Sirius had specifically told him to use it when he wanted to talk to him. And wouldn't Sirius at some point start wondering why Harry wasn't using the mirror and sending him a letter "Dear Harry, are you enjoying your christmas present? -Snuffles" to remind him of it? Or remind him when Harry talked to him after seeing Snape's memory?
2: Harry could have just gone to Snape, who's in the Order. Since this one is acknowledged in the book, I guess I can give Rowling a pass on it.
3: He could have asked a friendly seventh year student (say, Lee Jordan) to Apparate to Fred and George Weasley's joke shop and tell them to check on Sirius. They're in the Order too, after all.
4: Or he could have cut out the middle man and used the DA coins (Which they still had, as noted in The Deathly Hallows) to alert them.
5: When he broke into Umbridge's office to check on Sirius, why did he only stick his head in the fire, instead of just walking through it to look for Sirius himself instead of asking Kreacher? Walking through it would have three advantages: He wouldn't have to trust Kreacher's word, he wouldn't have to sit in an uncomfortable position, and if Umbridge unexpectedly returned to her office (Which she did), he wouldn't be caught.
6: When Snape pretended not to understand Harry's "He's got Padfoot in the place where it's kept!", Harry could have just realised Snape would say that whether he understood or not which Umbridge in the room. Harry should've just checked on Snape before rushing off to the Ministry after Umbridge got kidnapped by the centaurs. Even if Snape hadn't understood, Harry would've had a chance to tell him then, and since Snape can Apparate, he could've gotten to London much faster even if he had found Sirius had actually been taken there.
7: When they arrived at the Ministry, they could've realized Arthur Weasley and Kingsley Shlacklebolt work there, and asked them for help.
I guess that just goes to show why the series doesn't focus on a group of Ravenclaw students, despite Ravenclaw clearly being a much cooler house than Gryffindor.