2. Fitzgerald's Prose
This one is extremely simple and also a universal truth for all book to film adaptations. Nobody has ever been completely successful in transferring the style of a book to a film because they are different artforms so things have to be changed, there's no way around it. Fitzgerald's prose is some of the most elegant and beautiful that American literature has to offer but it can't possibly be translated to film. This goes beyond the narration and into the ways that Fitzgerald describes his characters in general. Take for example this bit from the book regarding Daisy's speech "Her's was the kind of voice that the ear follows up and down, as if each speech is an arrangement of notes that will never be played again." A film can't convey a statement like that because it goes against the nature of the medium. Film can show Daisy in all of her beauty and elegance but there's no way that it can illustrate that quote. An actress can only do so much but a book can build these characters into godlike figures and present these amazing worlds that can only exist in the reader's own head. Some books are tailored more for the screen then there are others, like this one and The Catcher in the Rye, that are quintessentially novels.