* In the novel, Fredo was big, but stupid; in the movie, he is small and stupid.

* In the movie, the heads of the 5 families are killed, with Rocco and Clemenza heavily involved; in the book, only Barzini is killed, with other killings happening as a result of the death. (The novel makes more sense)

* In the book, there is a bigger age difference between Sonny and Michael. They seem to be relatively close together in the movie, but the book implies they were fairly far apart in age.

* In the movie, a knife is stuck through Luca Brasi's hand to prevent him from moving while being garrotted, in the novel, he is just held.

* In the novel, Appolonia was pregnant, I don't believe that is mentioned in the movie.

* In GF2, the Vito back story ends after the killing of the Sicilian Don. In the novel, there is a lot of detail starting from the formation of Genco oil importing to the beginning of the 5-families war.

* In the novel, there is a scene at Genco's death bed that is not in the movie.

* The novel gives a lot of background info about Luca Brasi and Al Neri that is omitted from the movie.

* In the novel, Kay leaves Michael fairly early (right after the Carlo hit, if I remember correctly), but comes back later. Kay stays with Michael for at least the years between GF1 and GF2.

* In the movie, Michael's "Don't take sides against the family" line is to Fredo alone (with Tom). In the novel, Moe Green is in the room at the time (making the movie more powerful because of the foreshadowing of the betrayal in GF2).

* In the novel, someone cloned dinosaur DNA and created a theme park and . . . (whoops, wrong book)

I'm sure there's more, but I can't remember anymore off the top of my head.