Just… the treatment of Ginny in general
She could have been so much better, had Rowling given her a little bit of depth outside of being Harry Potter Fangirl #1. But for the level of importance we’re supposed to believe Ginny has to the series, nothing about her is actually specific to her. She’s an extension of Harry, a secondary character created to give depth to the main character, but without any real substance of her own. And she could have been amazing.