Ableism
I know the book was written in the early 90s when ableism wasn't a thing, but it's pretty lazy writing that someone with physical disabilities = bad guy. It feels like King could have just said he had a long face (like King has himself), unusually tall, and poor posture causing his arms to hang lower than normal. No need to bring a medical condition into it.
It's disappointing they didn't change this for the movie, especially considering that they did modernise some parts (wall phone to iphone) and made several plot changes (Jessie talks to Gerald instead of Ruth and the others).
Also as terrifying that someone who looks different can be, it's even more scary when the person looks like they could blend in anywhere. I feel like this would have better fit the theme of the book with her dad and husband able to hide the bad parts of themselves around others.