At one point or another, everyone has left a film and said, "the book was better." In the defense of the movie, how can they ever hope to fit a thirty six chapter book into only two hours?! They can't. Imagine if the first season of "Game of Thrones" was instead a movie. With ten episodes at about an hour each, you would lose about six episodes of story by making it a two hour film! That is exactly why Harry Potter needs to become a TV show. There was so much from the books left out of the films. The movies were great, but a TV show would have been amazing!
First off, the Dursley family was barely in the films, and had no interaction with Harry in the "The Deathly Hollows Pt.1". You never see Dudley finally be friendly to Harry. After Harry saves Dudley in "The Order of the Phoenix," he starts to realize that the Wizarding World cannot be ignored, and Harry is the only thing protecting his family. It's even hinted that Petunia doesn't necessarily hate Harry; she's actually just jealous of his powers. The film started to show this in the final film when Harry was looking at Snape's memories in the Pensive. The film showed Petunia freak out by what her sister could do, but left out the part when she sent a letter to Dumbledore asking if muggles could attend Hogwarts. If Harry Potter was a TV series, then we could see the Dursley family fully explored.
In the second book we are introduced to house elves, and one in particular, Dobby. House elves are enslaved by wizards and brutally torture themselves if they disobey their masters. When Hermione discovers how elves are treated in the Wizarding World, she creates a movement against it called S.P.E.W. (Society for the Promotion of Elfish Welfare). Spew is lightly mentioned in "Harry Potter and the Goblet of Fire," but not mentioned much more otherwise; however contrary to the movies, Hermione never gave up the push for elf freedom. Interestingly Hermione, being a mud-blood, came from the Muggle World and never experienced elvish slavery, but Ron grew up with it; so to Ron it's normal. Ron even makes comments that, "they like it." It somewhat mirrors American's slavery and the difference between someone being born in the North and someone being born in the South. Slavery isn't the only problem Dobby faced in the films. When it came to helping Harry, Dobby got the short end of the stick. In the books, Dobby is constantly helping Harry, but in the movies they seem to give the credit to Neville Longbottom. Dobby is the one that gave Harry the Gillyweed which turns him into part fish, allowing Harry to rescue Ron from the mermaids. Dobby is also the one that shows Harry the Room of Requirement; so that he and Dumbledore's Army could practice, Defense Against the Dark Arts, without interference from Umbridge. Dobby isn't the only elf not fully used either. Kreacher evolves as a character as well. Originally he hated Harry and the rest of the Order of the Phoenix, but in "Deathly Hollows," Harry begins treating Kreacher with a bit more kindness and Kreacher returns that kindness. Kreacher and Dobby often comically fight for Harry's approval. If Harry Potter was a TV series we could see elves represented properly.
Next is blood. Voldermort's entire war is predicated on "pure-blood wizards" ruling over all. Hagrid often ironically judges people based on their family. He often comments that the Malfoys have "bad blood." He tells Harry, "with a mum an' dad like yours, what else would yeh be?" So because Harry has his parents blood he will be great. This is a big theme in the Harry Potter books. Hagrid comes from giants, and giants are considered big, stupid, and dangerous. That's why they live far away in the mountains. He constantly talks about who someone had for a sister, or the family they came from, to determine how that person will turn out. What the movie left out is when Hagrid confronts Madame Maxime for being a half giant like himself, and she gets offended. The movie just has them become a happy couple. Hagrid argues that there isn't nothing to be ashamed off in the books. The films also leave out Hagrid and Madame Maxime's venture to the giants in the mountains asking for help against Voldermort. That's where Hagrid met his brother Grawp. Hagrid makes the argument that Grawp is kind once you get to know him. That's the point! When Harry is sorted, originally he was suppose to go to Slytherin, but asks to be placed into Gryffindor, and so he is. Someones blood doesn't determine the person that they are going to be. Each person can make their own decisions and determine their own outcome. There are so many examples of people being judged on their blood that is left out of the movies.
Dumbledore has a vast history that Harry uncovers throughout "The Deathly Hollows," but it was barely touched upon in the film. Dumblefore vs Grindelwald would be an epic battle that everyone would enjoy. What else is missing is how Dumbledor's sister died. Ariana was a half-blood wizard that was mentally scarred after being tormented by muggles. Dumbledore's father, Percival, sought revenge against the muggles and was placed in Azkaban until his death. Dumbledore's brother, Aberforth, and mother Kendra cared for Ariana until his mother fell ill, and Aberforth returned to Hogwarts. Then it was Dumbledore who cared for his sister. When Aberforth returned home for the summer, he found Dumbledore and his new friend Grindlewald learning dark magic and plotting to rule over the muggles. Yeah, Dumbledore was almost a bad guy! The movies skipped that bit. Also Dumblefore loved Grindelwald as more than a friend. Ariana was accidentally killed during a three way duel between Grindelwald, Aberforth, and Dumbledore. No one knows whose wand actually killed her. That is a television season in itself, but it barely dealt with in the films.
These few examples only scratch the surface of what the films left out from the books. That is why, "the book was better," is often said after a movie. There's simply not enough time. If Harry Potter was told as a television series then the audience would finally be able to get the proper re-telling of a fantastic and magically book series.