Besides the book being more graphic in it's portrayal of sex, violence, profanity, etc. I think the only big difference between the book and its film counterparts were the character personalities and character development. In the book, we get to learn more about Mama Corleone, Johnny Fontane, Lucy ... all characters who just come, linger a bit, and then go in the films. Mama Corleone, for instance, was a strong character in the novel. She instilled the virtues of a bona-fide Mafia wife in a Kay not as mouthy as the one in the movies. I should add that while the movie seems to be more family than Mafia, the book seems to be just as much (maybe more) about gangsters as it is about family.