As I've mentioned, this book gets a lot of undeserved hate. I've reread this book twice in the last year, and my love and appreciation for it keeps growing. This is possibly my favorite HP book (rivaling only POA), and I would argue it's the most important book in the whole series for the following reasons: 
 1. Harry is not an angsty, teenage-riddled mess. 
 He is having an appropriate and much needed emotional reaction to the trauma of watching his friend die right in front of him, watching the man who murdered his parents, and has essentially stalked him for years, come back to life, and barely escaped with his life ONLY TO DISCOVER a man he's trusted with his life has actually been a Death Eater the whole time. 
 Yeah, I'd be pissed too. 
 Prior to OOTP, Harry has maintained quite a bit of his innocence. Sure, he's faced a lot of harrowing trials. He came to Hogwarts as a damaged kid, and has faced more hardship than anyone should ever have to. But since arriving at Hogwarts, he has gained a lot of relationships and support he had never experience before. In fact, Harry is in a really good place going into GOF. He has a surrogate family with the Weasleys, a newly discovered godfather who becomes his brother/father figure, he has the unconditional loyalty and support of Ron, Hermione, and Hagrid, a mentor in Dumbledore, and a place he loves where he can study something that makes him truly happy. 
 GOF is when the reality of the hardship Voldemort represents sets in. Here is a tangible threat to everything Harry knows and loves. Here is something that is hellbent on killing Harry and changing the world order as it stands. Here is something pure evil. 
 When Harry comes out of the cemetery, he expects the world to believe him and ready themselves for Voldemort. He witnesses this horrible thing, and no one outside of his core group believes him. Even worse, they ridicule him. He waits to hear any scrap of news telling him that what he's seen is going to come to fruition, and no one tells him anything. He's forced to stay out of sight in the Muggle world cut off from everything he cares about. 
 You'd be pissed too. 
 Harry isn't an angsty mess. He's rightfully angry and rightfully traumatized. This is the book where Harry becomes an adult. This is when he takes an alarming amount of responsibility for himself without question. This is when Harry decides he's tired of being treated like a child for being in a war he was thrown into when he was a baby. He's just waiting for everyone else to catch up. 
 2. This book tackles dark themes such as death, abuse, corruption, and bullying. 
 A huge plot point in this story is the government's willful dismissal of Harry's claims. The lengths to which Fudge and Umbridge go to discredit and silence Harry are horrifying. There's a reason we all hate Umbridge more than Voldemort. She's a HORRIBLE human being, a fact that is never seen more clearly than when she physically tortures students with her quill and goes out of her way to mock and humiliate "half-breeds." Combine her character with the blatant bias of the Daily Prophet and Ministry, and you have a strong message that if your views don't align with the views of those in power, your name will be dragged through the mud at any cost. 
 What's brilliant about this book though is that Harry never once wavers. He loses friends. He faces daily doubt, rumors, and punishment. But he never recants his story. He continues to fight for his truth, and that's such a strong message to send to younger readers. 
 We also get a glimpse of severe bullying through Snape's worst memory and Luna. I won't get too much into the Snape side of things because it's a soapbox rant of mine, but I will say that given how much we know about the Marauders, which is next to nothing, it's ridiculous to base an entire opinion of James specifically on the memory of a man who is clearly bitter and biased. Yes, James clearly bullied Snape in that memory. It was awful, and no one should support that kind of behavior. Ever. However, memories are not unbiased. Snape has a lot of loathing for James, and that is much more complex and nuanced than one memory, especially given all the other information that has been hinted at throughout the series (such as the fact Snape hexed James at every opportunity; we didn't see a memory about that, did we?). I'm not saying James didn't bully Snape. What I am saying is that memories tend to tell one kind of story from one type of viewpoint. Jo has been very quiet about the Marauder's past (WHICH DRIVES ME CRAZY JO. I NEED TO KNOW MORE THAN YOU'VE GIVEN US), but she has said James and Snape's relationship is very much like Harry and Draco's. I'd bet money she's referring specifically to HBP with Harry and James and Draco as Snape. 
 ANYWAY, we see something very similar with Luna. Luna is a lovely addition to the series. She's an odd little dreamer. At the end of the series, we realize she's being bullied too because she's a little different than everyone else. However, Luna doesn't let this affect her. She is who she is, and she seems to love that. She embraces her own quirks. I'd like to think Jo introduced this in this book to show how differently bullying affects everyone. 
 3. Neville's backstory 
 I love Neville so much. His story broke my heart the first time I read OOTP, and it breaks my heart now. This is when Neville becomes more than the clumsy oddity in the Gryffindor house. He and Harry have much more in common than Harry realizes, and I loved seeing Neville begin to evolve into the warrior he becomes in DH. 
 4. The DA 
 I also loved the addition of the DA. This is when Harry, begrudgingly, takes ownership of his magical prowess. Of course, he doesn't see it as magical prowess but I do because this kid is impressive. I love the defiance against Umbridge, and the secret meetings to learn defense. I love seeing the kids work hard and take pride in doing something bigger than themselves. I loved seeing Harry in a teacher role (I'm still a little disappointed he didn't becomes the DADA teacher after Hogwarts but I'll get over it. Maybe). We got to see a different side of Harry that takes pride in preparing others for what's to come. 
 5. Loss 
 This is also the first time Harry loses something. Up until this point, Harry has only gained things/people that have bettered him. But this is the first time Harry makes a mistake, and it really costs him something precious. Honestly, this is the first time the readers lose something precious too. Cedric's death didn't hit home. It was a shock, yes, but most readers weren't attached to Cedric. Sirius, though. Sirius' death hurts. It hurts for us, and it hurts for Harry. We've grown to love Sirius for three books, and he's just gone. That leaves a tangible hole, and death to our favorite characters becomes a real possibility. The brewing war suddenly seems present and unavoidable. 
 Every time I reach Sirius' death, I basically cry non-stop until the end of the book. Jo does an amazing job at capturing deep grief, an emotion we haven't seen from Harry yet. We seen the rage but not loss. The scene in Dumbledore's office is one of my favorite scenes of all time for HP, not only because Harry just breaks my heart, but also because this is when Harry begins to see Dumbledore as someone who can also make mistakes. Dumbledore monumentally fails Harry. It's a big chapter of revelations, and there's no going back once Dumbledore drops the prophecy bombshell on Harry. This is a book of growing stakes for Harry. The amount of growing up he has to do in the period of a year is heartbreaking and astonishing. 
 Again, I know this book isn't a favorite among most HP fans but it's one of mine. This is such an pivotal book for Harry. Things weren't going to stay happy and cheery until the end, and I can't think of any other way I would want this book to go. 
 Reread Sept. 2015 - 12 years later, and I still cry miserably through the last 100 pages.