I read Ender's Game as a kid and really liked it. I didn't think it could work as a movie because 1) they couldn't cast enough children who wouldn't come across as just being ridiculous and 2) they would be skittish about children being killed.
But then Hunger Games came out and it seemed like everyone was okay with children being killed now. Hooray! As for the other thing, they just cast older kids, which I thought worked out okay.
Actually, given all the ways they could have totally screwed up the movie, there was really only one thing that I felt majorly changed from the novel, which was the kids at Battle School were genetically engineered and that's why they were all so weird. Having googled this topic, there are a lot of people saying that none of them were actually genetically engineered (perhaps selective breeding at worst) except for Bean, but they were just all so weird. Like, there is raw intelligence and that can certainly make a kid different, but that just doesn't change certain fundamental aspects of humanity, and intelligence doesn't necessarily imply the type of analytical, strategic or tactical thinking the kids displayed. And that didn't come across in the movie. They were just bright kids.
The trailer spoiled the entire movie, LOL. And it led me to think that they would not follow the book in a very important way, but they actually did. So the trailer screwed up, not the movie. And they included some stuff at the end which was really important and sort of the point of the entire book, but I thought they might not, because it seemed like a bit too much thinking for an action movie.
To be explicit, [Spoiler (click to open)]the trailer made it seem like the kids knew all along that they were directing the human battle fleet. In the movie, the kids think they're playing a simulation (like in the book). At the end, Ender finds the structure from the game he's been playing and finds the Formic egg. They actually address the issue of xenocide and how maybe they shouldn't have done what they did. They don't explicitly say that the Formics would never have invaded again, but I think they showed enough in the movie.
Overall, I enjoyed the movie. I really liked the design of the glorified laser tag room in Battle School, although the overall feel of the station seemed small. All of the Formic stuff was absolutely amazing, from the design of the Formics themselves, to their hive ships and swarms and their alien structures. The child actors all did pretty good. I was a little weirded out by Petra being flirty with Ender, but she's like 10 years old in the book and she's about 15 here, so I suppose that's okay. I would have preferred kids to be killed in the movie, but *shrugs*
It's tough to think how I would have liked the movie without having read the book though. There were a few places where I think I might have been confused or thought something was stupid if I didn't have knowledge from the book. It's weird, I've always thought of it as a short book, but now I realize how much there was to cram in. There was enough material for a miniseries!