I am not going to talk shit about the movie because it was amazing... it just wasn't as good as the book.
For four main reasons:
Some characters were left out. Was it only me who missed Kaitlyn (Hazel's friend) or Gus's ex-girlfriend? I know Kaitlyn didn't contribute to the book that much but she acted like Hazel's wing man in a different sort of way. Kaitlyn was always giving her advice on Gus and Amsterdam. I just hoped she would be in the film is all. Gus' ex-girlfriend, like Kaitlyn, didn't play a big role but that doesn't mean that you should or even could cut her out of the entire film.
My second reason is that scenes from the book just simply weren't in the movie. Most scenes were irrelevant but a few should have made it into the movie, for sure. An example being the part where Hazel goes to the mall with Kaitlyn and ends up telling her that she has to get going after only 30 minutes of hanging out but stays and reads by herself instead. A little girl comes up to Hazel and treats her like an ordinary person who doesn't have lung cancer, and she realized that only little kids and their perfect innocence would act like they are somewhat the same.
Hazel doesn't die in the movie. Sure, they never really say it in the book but John Green kind of hints about it from the very beginning. Then the movie ends so abruptly. Hazel basically just reads the letter left for her, lies down on the grass trying to find comfort and says "okay," before the credits roll up and Ed Sheeran begins singing "All Of T111he Stars."
And finally, probably the biggest reason for me is the whole relationship between Augustus and Isaac is played down. I know there wasn't that many scenes of them together in the book as the book was from Hazel's perspective and nothing she couldn't see, touch or hear could be a part of the book... but that's the great thing about movies, you don't have to stick to a one person's point of view.
Anyway, as I was saying: I think they really should have included the scene where Hazel and Isaac are playing video games after Gus' untimely and painful death, he turns and says to the computer "I don't want to live in a world without an Augustus Waters." That scene just shows how close they really were and it is absolutely heartbreaking.
Again, don't think that I am saying anything bad about TFIOS film. It is a fantastic movie and I loved it. But in my opinion the book seems better.