I actually enjoyed this adaptation. While there some changes here and there, they kept true to the themes of the story: loss, love, family, friendship and all that in between. I only have two things I wish they movie maker's didn't do:
1) Reduced the family interaction. There were two scenes in the book I loved that were no present in the movie. The dinner scene with Hazel's and Gus' parents and the scene with all of Gus' sisters. Both scenes have funny moments as well as heartbreaking moments and it was great to see how the families were dealing with the tragedy of the kid's having cancer. Especially, Gus' family at the end when he gets sick.
2) The Limo Scene. This was not in the book. Instead, this scene replaced the one where Hazel and her mom go to Gus' house and overhears an argument happening. It was the first time Hazel saw Gus as anything but optimistic. It was the first time the readers saw that and I always felt it was a good hint that even the most optimistic people have moments of doubt and conflict.
Not having either of these scene didn't deter from the story that was told, but it would have been nice to see them.