One of the main differences that I noticed was about Jonas. In the book he was said to be 12 years old, while in the movie he looked 18 years old. One thing that was the same was his dark hair. I always imagined him with dark brown hair. In the movie he has dark brown hair.
Another difference that I noticed was how the memories went back to the people. In the book the memories gradually slipped away. In the movie, he had to cross a boundary to give all the memories back to the people. In both the movie and book, he was in cold weather when the memories went fully back to the people.