This is the hard one. 
 This is the longest Harry Potter-novel, and it is the hardest one to finish. Its many pages doesn't work in its favor. I remember my sister giving up on it, even though she had loved the series so far. She simply couldn't make it past the angry main character, screaming in Caps Lock, the long summer without answers and the many discussions of politics. She wanted to read about Hogwarts; about spells, magical creatures and adventures. 
 And she wasn't the only one. 
 The main problem is the pace. This book paves the way for book 6 and 7. And while I appreciate Rowling taking her time to build up her finale, it doesn't work for me at all. This book is filled with more questions than answers - and most famously, I hate the way the thestrals were handled. 
 (view spoiler)[Thestrals can only be seen by someone, who has watched someone die. One could argue that Harry should be able to see them from the get-go, but Rowling has made it clear, that he didn't fully comprehend what was happening to his parents as he was only a toddler, and therefore wasn't able to see thestrals. However, all that changes when he watched Cedric die in book four. 
 But(!) shouldn't he then be able to see the thestrals, when he leaves the school in book four? Apparently not. 
 Rowling says: "If Harry had seen them then and we hadn't explained them then, I thought that would be rather a cheat on the reader in that Harry suddenly sees these monsters but we don't go anywhere with them, so to explain to myself I said that you had to have seen the death and allowed it to sink in a little bit before slowly these creatures became solid in front of you." 
 The thestrals are a product of convinience. Rowling made up the clumsy rules (which are never fully explained in the books, leaving many readers puzzled), because she didn't want to explain the thestrals in the ending of a book. The solution she came up with is far from elegant, it is so contrived, making you extremely aware of the writer behind the book, pulling the strings and arranging the words in order to make the story flow nicely. (hide spoiler)] 
 While I understand why Harry is angry, and while I understand why he needs to be isolated, I feel like Sirius was done a major injustice in this book. His restlessness is of course understandable, but his need to encourage Harry to trouble and danger is so unlike him. 
 Sirius spent the entirety of book four warning Harry about upcoming dangers, urging him to be selfish and only focusing on getting safely out of the tournament. But in this book, Sirius suddenly becomes irresponsible, urging Harry to act out instead of being safe. Why? The man spent half of his lifetime in Azkaban - I would think he had learned some patience. Instead, a few months in hiding turns him into a madman. 
 This book suffers from its length and from Harry's isolation. It becomes quite tiresome - though I of course always loved the idea of Dumbledore's Army as a way for the younger generation to mimic the Order of the Phoenix.