1. Gatsby Himself
This is another one that is more inherent to the nature of novels themselves. In the book Gatsby is built as this mythic figure then slowly humanized until he is the living embodiment of the American dream and all of it's repercussions. The films don't follow that trajectory and instead present Gatsby as this larger than life caricature of who he is in Fitzgerald's novel. Baz Luhrmann did a decent job of bringing Fitzgerald's Gatsby to life on the screen but DiCaprio also deserves a high amount of praise in that regard. There are still areas where Gatsby was portrayed as kind of a goofy/cartoonish character that betrayed the nature of his being in the novel but overall Luhrmann was far more successful than anyone else in making Gatsby a real human being. The fact still remains that it is nigh impossible to represent the Gatsby of Fitzgerald's prose since he is more than just a character. He is the very embodiment of America in the 1920s and the excesses of the Jazz Age. There only comes a work of art about once a decade that features a character who is a representation of an entire generation and ideal. Think Charles Foster Kane as a representation of wartime America, Don Vito Corleone as the embodiment of the dark side of the American dream and the immigration experience or Daniel Plainview as a placeholder for capitalism and greed at the turn of the century. Fitzgerald presented that character in literary form and he just can't be re-captured on film after being so perfectly realized in the book that bears his namesake.
Thanks so much for reading. I'd love to hear your thoughts and opinions in the comments.