20 Biggest Things The Harry Potter Movies Left Out
8. Aunt Petunia's Correspondence
In The Book
In the books, Dumbledore and Petunia sent a number of letters to one another. As a child Petunia sent a letter to Dumbledore requesting that she too could go to Hogwarts like her sister, to which the headmaster replied kindly, letting her down. She was wounded and cut off her contact with her sister and the Wizarding World as a result of being left out.
Then later, after the Dursleys entertain the idea of throwing Harry out when they know Sirius Black is looking for him, Dumbledore sends a howler, chillingly shouting that Petunia should "remember my last", and implying something he'd written to her told her how crucial it was that she kept Harry there. Interpretations vary on what it precisely meant, but at least it happened.
In The Film
Only one letter appears, in Harry's basket when he's dropped off at Privet Drive. No Howler, no backstory for Petunia.
Was It A Positive Cut?
Not really - the correspondence in both cases adds a lot to Petunia's character, and added a personal tragedy to her rejection of the Wizarding World in the first case in particular. She's less of a monster (fitting in with the image that almost nobody in the books is what they seem to be).
And in the case of the Howler, it explains why the Dursleys would keep Harry, despite their outright hatred of him. It also vaguely entertained the idea that Petunia actually had some maternal feelings for Harry, despite herself, which again made her a lot more interesting.