The movie got far more right than it got wrong. The two most important performances in the movie -- Asa Butterfield as Ender and Ben Kingsley as Mazer Rackham -- were exactly right. A lot of the stuff with the interpersonal dynamics at Battle School they captured very comprehensibly while condensing considerably. I didn't think the Petra character was particularly well-served by the movie. This is less a criticism of Hailee Steinfeld's performance than of the role the character was given to play in the film. She basically served as a Valentine surrogate for much of the film, which made her a lot more emotional and maternal than the character on the page. You never get that sense of her as the competition like you did in the book. As a result, you don't get that very powerful beat where Ender pushes her too far and she breaks.
While I didn't particularly miss the "Locke" and "Demosthenes" online essays, which we can assume occurred off-screen, I did miss the sense that Peter and Valentine were just as brilliant Ender, only one ran too cold and the other ran too hot while Ender hit the Goldilocks place of just right. In her handful of scenes, I thought Abigail Breslin actually did a great job of capturing Valentine. But I think Peter could have used a few more beats to flesh him out a bit. He's a complete psychopath, but he's a very different kind of pyschopath from Bonzo and I don't think the movie captured that.
Harrison Ford isn't what I pictured at all for Colonel Graff, but it's one of his best performances in years. In the book, he's a sort of Taft-like spidery man of cunning and guile. Ford's version looks like your typical movie military man hard charger, but in a way that's to the movie's benefit. It helps mask the cunning and guile, which are still very much there, and makes some of his manipulations of Ender a bit more unexpected. Most importantly, the dynamic between Ford and Butterfield completely works on screen.
The movie loses something with the necessity of having a twelve-year-old pubescent Ender whose experiences at Battle School encompass a matter of months rather than a six-year-old who is literally raised at Battle School over a matter of years. Some very obscure details are immaculately captured, like Ender's father's eastern European accent and Mazer's New Zealand accent. Others sort of quizzically fall by the wayside; while the racial composition of the Battle School candidates remains admirably varied, judging by the kids' accents you'd think they were all American. Alai should have an accent from Northern Africa. Bean and Deek should have Dutch accents. While the book isn't clear of Petra's country of origin beyond her Armenian heritage, the Bean sequels make it clear that she should have an Armenian accent.