7. Why does Malfoy try to befriend Harry when they first meet?
At their first meeting, Malfoy tries to persuade Harry not to be friends with people like Ron, and, instead, to be friends with Malfoy and his cronies. I get that Malfoy would likely want to be friends with the celebrity student, but his father was a supporter of the person who tried to kill Harry, so it's a pretty weird stretch to think this relationship will go well. Even if the elder Malfoys think Voldemort is dead and don't still have a vendetta against Harry, there's going to be some bad blood, so I don't understand Draco's endgame.
8. Why aren't they more careful what they teach Slytherin kids?
A lot of people might suggest scrapping the Slytherin house all together, but that seems excessive. After all, not every Slytherin kid becomes evil, BUT, apparently, every evil wizard was in Slytherin. So, obviously, Hogwarts should be doubling down on ethics lessons for these kids. Maybe, instead of teaching them so much about fighting, try to prevent the obviously evil children from becoming more evil.
9. Why would Harry think he could get away with opening a letter in front of the Dursleys?
It's presumably the first letter he's ever gotten and he just goes ahead and opens it in front of his awful family? Idiot.
Now for some complaints that are just about the movies:
10. Why was Moaning Myrtle played by a 37 year-old?
Moaning Myrtle is played by Shirley Henderson in the movies, and, if the name "Shirley Henderson" didn't tip you off, this woman was way too old to play a teenager in 2002. Granted, Shirley Henderson is a very good actress and has a great, horrifying voice, but it's still really weird they cast such an old person to play a dead teenager. If the casting director can explain this choice to me, I'll strike it from the list, but until then, it stays.
11. The Ron/Hermione kiss was the lamest kiss ever.
The entire series--even starting when they're 11 years-old--builds the romantic tension between Ron and Hermione. We wait the whole series for them to get together, and then, at last, in the final battle, when people have died and more lives are at risk, our two star-crossed lovers unite and we get to see the back of Rupert Grint's wet head. Be still my heart.
12. And, finally, the most bothersome thing in the entire movie series: the actress they get to play the young version of Harry's mom doesn't have eyes that look anything like his.
So, let's work through this. It's the final installment of a multi-billion dollar movie franchise based on a multi-billion dollar book franchise. Throughout the series, characters are constantly talking about how Harry Potter has the same eyes as his mother. This one fact is the thing that makes Snape turn his back on the Death Eaters and try to protect the child of his one true love. Lily Potter and Harry Potter having the same eyes is a huge fucking deal. So, when the filmmakers needed to cast an actress to play young Lily Potter, a role that required no actual acting skill, and really JUST existed to show her resemblance to Harry, who do they cast? A girl with brown eyes that look nothing like Harry Potters. Good work, everyone.