1. Hermione has big, buck teeth in the books, but not in the movies. Harry is also supposed to have green eyes. According to the movie's 'Trivia' section on IMDB.com (*link below), both of these were attempted, but Daniel Radcliffe's eyes had a 'strong reaction' to the contacts, and Emma Watson couldn't speak clearly with the fake teeth in.
2. Before going on to Voldemort, Harry and Hermione go through the door with Wizard's Chess and into a room filled with potions. The exits are closed off by flames. One potion will get you through the next door, one will get you through the one you came from, and one will kill you.
3. Dudley's friend, Piers, comes with Harry and Dudley to the zoo, and tries to explain to Uncle Vernon and Aunt Petunia that Harry was talking to the snake.
4. Harry goes to the zoo with them in the first place because Ms. Figg, who usually babysits him, has broken her leg.
5. Ms. Figg is not even brought into the movies until the the 5th one.
6. Aunt Marge is mentioned here; when they are trying to find a substitute babysitter for Harry, Aunt Petunia suggests Aunt Marge, but Uncle Vernon says that no, "she hates the boy".
7. Harry is Dudley's favourite punching bag, and Dudley's favourite pastime is "Harry Hunting".
8. Harry remembers odd instances which turn out to be magic; one is where he accompanies Aunt Petunia to a grocery store and an odd little man shakes his hand; one is where she practically shaves his unruly hair, and it grows back to normal by the next morning; one is where he tries to jump around the dumpster in order to get away from Dudley's gang, and winds up on the roof.
9. Dudley's Second Bedroom, which Uncle Vernon relocates Harry to when the letters he is sent are adressed to the Cupboard Under the Stairs, is left out of this movie. However, in Chamber of Secrets, he is clearly living in the said bedroom.
10. There is a scene in the book in which Harry tries to sneak downstairs to get a letter, and winds up stepping on Uncle Vernon, who had a similar idea.
11. Uncle Veron gives Harry a ride to the train station, because they need to have Dudley's tail removed anyway. When they get there, there is (obviously) no sign for Platform 9 3/4. Uncle Vernon merely smiles nastily and tells him to have a nice term, and leaves.
12. Although in the movie Hagrid immediately takes Harry to Diagon Alley after the confrontation with the Dursleys, he actually has some food, some tea, and waits until the next morning; they then take a boat and then a train to get there. There is, however, a deleted scene depicting them on a train.
13. Harry tries to tell Uncle Vernon about a dream he had, about flying motorbikes. Uncle Vernon shoots this down by telling him that motorbikes can't fly.
14. Most of the first chapter is left out; the part where Dumbledore leaves Harry on the Dursley's doorstep is there, but the part following Uncle Vernon's day at work is not.
15. Harry actually meets Draco Malfoy for the first time in Diagon Alley. They are both getting fitted for robes. Malfoy makes a snide remark about Hagrid and tries to talk to Harry about houses and Quidditch, but Harry does not understand what he's talking about.
16. After this, Hagrid tries to explain the houses, Quidditch, and Voldemort to Harry.
17. Harry and Ron are supposed to go duel with Malfoy. Hermione follows them out of the dormitory, telling them they shouldn't be going, and gets locked out because the Fat Lady has left her picture. Neville also winds up following them because he gets locked out as well. This is when they all meet Fluffy.
18. Peeves the Poltergeist is completely left out. Most of the ghosts are as well; the Fat Friar and the Bloody Barron are seen in the beginning, after the sorting; Nearly Headless Nick is not incorporated into the movies after the Chamber of Secrets.
19. Harry and Ron meet Neville before they meet Hermione; he comes looking for Trevor as well.
20. Malfoy, Crabbe, and Goyle also go to Ron and Harry's compartment. They are intending to steal sweets, but are attacked by Ron's rat, Scabbers. Hermione comes in afterward, hoping that they weren't fighting.
21. Harry, Ron and Hermione do not get detention for visiting Hagrid after hours, and the staff does not know about Norbert.
22. Harry and Hermione get in trouble when they are caught coming out of the Astronomy tower after hours. They were there delivering Norbert to Ron's brother, Charlie, and left the Invisibility Cloak up in the tower. Neville gets detention along with them; he was out of bed trying to warn them about Malfoy.
23. Neville and Malfoy are originally paired in detention, but are separated after Malfoy scares Neville, who sends sparks flying.
24. More than one centaur is present in the forest scene; they both speak cryptically about the stars.
25. In the movie, Dumbledore never explains that the reason Snape hated Harry's dad so much was because he saved his life, and Snape couldn't bear to be in James's debt. Later on, it is explained that Sirius had tipped Snape off about Lupin being a werewolf. Snape, going to catch him in the act, was on his way, when James stopped him and saved his life.
26. Harry actually sees Snape's dog bite when he goes back to the library to get the book Quidditch through the Ages - given to him by Hermione and taken away because library books are not allowed out of the library.
27. Another one of the things guarding the Stone is a troll, but it has already been knocked out.
28. They aren't able to sink through the Devil's Snare; Hermione climbs through it and sets fire to it.
So here's all the stuff they left out :D
