So many of the characters make really, really stupid decisions. For example, Albus decided to risk his life and his best friend's life (who, may I add, wanted nothing to do with this), all to go back in time and save Cedric Diggory. Because he heard Amos Diggory getting upset at Harry for not using the Time Turner they found to save Cedric, Albus decides, "Screw it, I'm going to force my friend to break into the Ministry with me, break a crap-ton of laws and steal a Time-Turner from the Minister's office, to save this kid I never knew." Oh yeah, and Cedric Diggory is the key to them winning the war. Albus and Scorpius try to humiliate Cedric out of the Triwizard Tournament so he wouldn't touch the Cup, but they humiliate him so much that he becomes a Death Eater and kills Neville Longbottom, so he never kills Nagini, so Voldemort wins. What I want to know is: why didn't Albus and Scorpius go back to the Third Task of the Triwizard Tournament, inside the maze, and Stun Cedric so he didn't touch the Cup? He was happy to give the Cup to Harry, after all, surely just losing wouldn't make him evil, at least without the humiliation part. Why didn't they do that? In fact, why didn't they just go back in time and kill Voldemort when he was a kid? Not only would Cedric have survived, but Harry's parents and countless other people would have survived.
Also, Hermione is the Minister for Magic, and she is keeping this last, illegal Time Turner in her office. Instead of using a powerful enchantment to hide it that is nearly impenetrable, she hides it in a book in a bookcase, with each book saying a riddle that will help you figure which book it's in, and it was easily beaten by a couple of fourth years. I thought Hermione was smart?
Also, the rules of Time Turners are completely different than they were in the third book. In Prisoner of Azkaban, it's shown as a closed loop; all the things that happened when Harry and Hermione went back in time already happened, such as Harry producing the Patronus. In this, they can go back years and years (I just keep imagining them spending ages working out exactly how many hours back they need to go, considering they're travelling over 20 years back in time and they can only be in a different year for five minutes or they'll get hurt). They then get sent back to the present time when they're ready and major things will have changed.
The villain of the play also writes her plans on her wall. I'm not kidding, her plans and motivations were written on the walls, so the heroes could easily find out all they needed to know about her. Also, when she gets captured at the end, she immediately gives up and begs for them to kill her and wipe her memories. I thought you wanted Voldemort to be proud of you?
You're wondering why she wanted Voldemort to be proud of her? Because she's Voldemort's daughter. That's right. This is the one thing that pissed me off the most about this book. Without this twist, I wouldn't have liked the book, but I at least wouldn't have been angry about it.
Apparently, the villain is the daughter of Voldemort and Bellatrix Lestrange. She was born in Malfoy Manor before the Battle of Hogwarts. First off: Draco is on the good side in this novel, so how did he not know that a) Bellatrix was pregnant, and b) that there was a baby in the house? Second off: why would Voldemort do that? The only logical reason I can come up with as to why he would do that is to try and create an heir to ensure his survival. Here's the thing about Voldemort, though: he is extremely arrogant. Every time he has Harry in his grasp, Harry escapes because of the stupid rule that only he could kill him, and his incessant monologuing. Despite this, Voldemort continued to monologue and continued to insist that only he could kill Harry, and every time he caught Harry he thought he'd succeed in killing him. He also thought that no one would find out about his Horcruxes, and even if they did they wouldn't know where to find them, but not only did Regulus, Dumbledore and Harry work out his secret, three teenagers managed to find all of his Horcruxes. He only knew that they'd found his Horcruxes after they broke into Gringotts, which was a day or two before the Battle. That's the only time I imagine he would've thought an heir was necessary, but there wasn't nearly enough time to impregnate Bellatrix, considering they both die at the Battle of Hogwarts.
I can deal with everything else being canon, even though I don't really like it. But the fact that they gave Voldemort of all people a secret daughter just angers me.