So I've just finished reading Chuck Palahniuk's 'Fight Club'. I hadn't previously read the novel but am a big fan of the movie, which stars Brad Pitt and Edward Norton. In creating the film they stuck very closely to the book's script (which was proabably relatively easy as it is a slim 200 page paperback) and this only makes it more interesting to see where the divergences are and what effect they have on the overall experience.
1) Tyler's Dream - The final scene of the movie, as the skyscrapers explode to the sound of the Pixies 'Where is My Mind?' is a brilliant climax. It is a remarkably upbeat ending, since the narrator has 'killed' Tyler Durdan and has achieved his dream of financial revolution against the debt agencies and major banks. While the methods are crude and completely insane, I think many people would have some sympathy with such a redistributive motivation.
The novel provides a much darker presentation of this dream. Tyler's visions of hunting elk through the ruins of North American cities evokes the desolation of 'I am Legend' and suggests mass death and destruction in an apocalyptic end to civilization. The narrator is open about the roots of this dream in his personal envy. He wants to destroy the 'beaches of France' because he will never see/possess them. Instead of destroying the skyscrapers through some altruistic dream of freeing people from capitalistic domination he wants to use them to squash the images of the past contained in museums. Small details but they remove any sympathy you might have for his actions.
2) The Ending - The upbeat nature of the movie's cliffhanger ending is not maintained at the end of the novel. We continue the story after Tyler's execution to find that the narrator has been hospitalised. This location supports your pre-existing doubts about the unreality of the narrator's story but, assuming that it was broadly true, we are left with the continued menace of the Project Mayhem operation, which is actively working to retrieve the narrator. A sword of Damocles is left hanging over him and modern civilization.
3) Greater Unpleasantness - The scenes from the novel that did not make it into the film are exactly those that present the narrator/Tyler in their worst light. The tipping of a rich woman into a suicidal depression through deceit, the murder of a political investigator of Fight Clubs and the eventual killing of the narrator's long-suffering boss are all deeply ugly scenes. Their omission from the film enables the misbehaviour of the protagonists to be initally presented in a far more comedic manner. Tyler plays with nunchucks and engages in banter with the narrator rather than being the hard-headed zealot of the novel. This gives a markedly different feel to the film despite its superficial similarities to the structure of the book.