It’s been said, but the age of some actors really didn’t line up with the books/the images in my head. Whenever we see James and Lily, their deaths should be obviously tragic because they should look like 21 year olds - barely out of school and some of the firsts of their year to start a family. And that should be more and more obvious as Harry ages so when he sees them again “at the close” they should look more like peers than parents. The same is true for everyone of their age group. As much as I love Alan Rickman, and firmly believe that he had the perfect voice and mannerisms for Snape, he was far too old. And so were the actors for Remus and Sirius.

In terms of the younger generation: - Harry’s hair was never quite right. It should be messy and wild. And he was missing some of his spark; Harry in the books is sassy and can be quick to anger. Also, his eyes should be unmistakably green. - Ron - and really all the Weasley boys - didn’t look anything like how I imagined him. Ron should be taller and leaner and fitter, like a soccer player. I also see the Weasley hair as a brighter orange.
- Neville was supposed to be blonde.

In terms of the physical world: - The Hogwarts quidditch pitch in the films makes no sense in my mind. Instead of a bunch of viewing towers, I saw it like a regular stadium - only elevated higher into the air. More like what they did for the World Cup.
- The world in general was brighter and the colors were more vibrant and whimsical - ie, Dumbledore’s fashion should border on ridiculous; Tonks’s hair should be brightly neon; and WWW should blind you with color. (Personally, I see the store as an obnoxious purple, but I don’t remember if that’s canon.) I feel like the films went for a dingier, darker palette.