Let's get down to some important changes missing from Sorcerer's Stone. (PSSST. Did you know Columbus intended to make the films longer but test audiences were against the idea?!)
Both in the book and movie, Dumbledore and Professor McGonagall with Hagrid leave baby Harry on the Dursleys doorstep so he can lead a normal life until he's ready to find out about his background. What the movie overlooks is how the wizarding world discreetly unveils itself to the boy who lived. Before Hagrid reveals to Harry he is a wizard, witches and wizards discreetly try to praise Harry while out running errands with his aunt, uncle, and cousin. Harry's abilities, though unknowing that they are magical, occur before he sets the Burmese python (in the book a boa constrictor) loose at the zoo - after a haircut he made his hair regrow immediately, transported himself onto the school's roof when Dudley was teasing him, and shrank a sweater.
In the book, Harry meets his Slytherin rival Draco Malfoy at Madam Malkin's robe shop in Diagon Alley. Malfoy's sharp and rude attitude makes Harry uneasy to extend a friendship. This takes place mostly between the two of them without an audience or authoritative figures. However, in the movie, Harry has a full audience of all the other Hogwarts students when he chides Malfoy about not making friends with the wrong sort. This change could be argued that it works because Harry one-upping Draco in front of his peers stirs jealousy and he later tries to humiliate Potter on multiple occasions which inevitably backfires.
Among the impressive special-effects that come to life when the students are first being sorted, the occasion itself is compressed in the film. The Sorting Hat performs a song before the school. Only some of the song is included in the Harry Potter and the Goblet of Fire adaptation.
When Neville was on-screen, he certainly made an impact - which is why everyone will agree there was too little of him throughout the movies. The 'lack of Longbottom' tradition began in the first movie when his role was minimized to forgetting his rememberall and standing up to the golden trio before they attempt to retrieve the Sorcerer's stone.
In the book, Harry and Draco's beef gets so heated they attempt to duel pass curfew. Hermione tries to stop Harry and Ron, and Neville is forced to tag along when he is locked out of the Gryffindor common room. The duel was a rouse to get Harry in trouble with Filch. SO, Neville is actually with the trio when they discover Fluffy. He also subsequently faces detention with Harry, Hermione, and Draco as punishment for visiting Hagrid after hours, but the movie substitutes him with Ron.
Hagrid warns Harry and his friends in the movie, "Snape is one of the teachers protecting the stone, he's not about to steal it." But "the how" doesn't necessarily come to light. Armed with assumptions about the Slytherin headmaster, the threesome try to retrieve the object before "he" does. Hogwarts teachers set in place different challenges - the devil's snare by Professor Sprout, charmed keys by Professor Flitwick, and the chess game by McGonagall. The movie leaves out Professor Quirrell's obstacle (a troll), and Professor Snape's challenge, which is a logic puzzle involving potions - one that Hermione brilliantly solves.
- Harry's grandparents do not appear at all in the Mirror of Erised in the film.
- Hagrid reveals Harry's a wizard on his twelfth birthday July 31st, but he has to wait until September 1st to travel to Hogwarts. In the movie it can seem like it happens overnight.
- Peeves the Poltergeist loves to cause trouble for Hogwarts students, but he never makes an appearance in the films.
- Miss Figg is Harry's babysitter when he's younger. She is not mentioned in the first film, but she later testifies on his behalf to the Ministry of Magic in Order of the Phoenix popping up as one of the neighbor's Harry knows/is an eyewitness to the dementor attack.