Left out of the film but were in the book:
1) It doesn't show Lennie having his illusions of his Aunt Carla and the rabbits when he was waiting at the pond.
2) George doesn't hesitate to shot Lennie at the end of the novel like he does in the book.
3) Candy never comes into the room where Crooks and Lennie are talking to each other. This is important because Crooks never found out that the plan about the little house was real.
Things in the movie that weren't in the novel:
1) Curley's wife is attracted to George.
2) George is only portrayed to dislike Curley, while in the book, he loathed him.
Hope I helped.