The biggest plotline that was cut from the films that still has me absolutely shaking in fury is the relationship between Tonks and Lupin. She doesn't say a word to him in Order of the Phoenix, then she's calling him "sweetheart" all of a sudden in HBP, then she's calling him her husband in DH1, then Lupin mentions his son once in the last movie. One of the most beautiful, complex, heartbreaking couples, and they get reduced to a little throwaway line in each film. The fact that this was cut from the films was inexcusable, and I'm at a loss as to why Rowling allowed it, since she said Lupin was one of her favorite characters.