NonModern has already had one take on this movie in which the faithfulness to the book was applauded. In 2001 when this film and the first of the Lord of the Rings movie were about to be released, there was a lot of nervousness in fandom. Hollywood’s general practice in treating books is to butcher the story. Harry Potter and the Sorcerer’s Stone stayed pretty faithful to the plot and story of the book—for a Hollywood film.
In reality, the differences are many if slight. However, some things have been greatly changed and in ways that really mean the viewer misses aspects that the reader witnessed—aspects that were very important.
For example, one of the biggest changes in the film is the whole dragon sequence. In the book, Harry and co. help Hagrid care for his illegal pet while trying to convince him to get rid of it. They plan, together with Ron’s brother, to have the dragon smuggled away from the castle one night. Harry and Hermione (but not Ron because he is sick from a dragon bite) take it to a high tower after hours and see it off. Neville finds them to warn them that Malfoy is trying to get them caught. In the end, all five students are punished for being out of bed after hours.
The important aspect of this original sequence is that for a long time after this, Harry and co. are extremely unpopular because they have cost their house a lot of points and the lead in the house cup race. They broke the rules and face severe consequences. When you know this, Neville’s actions at the end make a lot more sense. Also, you see how serious Harry is at the climax of the book when he is forced to break the rules again.
The battle of Good vs. Evil is serious. It is not about obeying a set of rules that will keep people happy and make you popular or acceptable to a society. The series will go on to have a lot to say about appeasement and following the status quo, but here we see that Harry and his friends know what the stakes are. It is not about staying safe and not getting into trouble with the authorities at school. It is a case of life or death. While the movie makes the tests the kids face in the end more cinematic, it misses much life-lessons taught throughout the book.
One thing that the movie did do a good job of highlighting, that can be skimmed over in the book, is the self sacrifice of Ron in the chess game. This is just one of many cases (and not the first even) where we see that Rowling identifies self-sacrifice as the supreme sign of love. The good guys will face death for love; the bad guys avoid death at all costs. More on that as the series progresses.