One of the reasons I enjoyed the book better than the movie is because of the sky disappearance. The sky disappearance is a big part of the story because it makes the story more tense and urgent.
The death of Alby was more emotional in the book. This is so because Alby sacrifice himself so the Gladers can get through the griever hole. In the movie he is yanked out of the homestead.
The connection between Thomas and Chuck in the book is better in the book than the movie. this is so because Thomas thinks of Chuck as a brother in the book. But in the movie Thomas never interacts with Chuck.