Basically All of Ginny's Humorous Scenes
Let's be honest here, the Harry Potter movies did an awful fucking job of representing Ginny. If you haven't read the books, you basically just see Ginny tagging along with her siblings sometimes and being a bothersome little sister. However, if you've taken the time to read the books, you know that Ginny Weasley is a totally rad character. She's funny, got a fiery temper, can cast one hell of a bat bogey hex, and she's a stunner.
As you probably remember, Winky was Barty Crouch Sr.'s house elf. She played a big part in GoF, but she was totally cut from the movie, as was Dobby. Not only do we miss seeing Dobby and Winky's amusing interactions with one another, but we also miss the part where she is accused of casting the Dark Mark.
During Harry Potter and the Half-Blood Prince, Dumbledore and Harry observe a memory where we visit the House of Gaunt. Here we meet the grandfather, uncle, and mother of Lord Voldemort. In this memory, we learn a lot of important things about Voldemort's family. In later memories (which were also cut from the film), we also learn about Lord Voldemort's father as well.
These are just a few of the many scenes that all of us wish had taken place in the movies. In my opinion, more of the movies should have been split into two parts because there really are some important things from the books that was totally cut from the movies.