From the previews, I expected that Ender's Game the movie had taken major liberties with Ender's Game the novel. After seeing the movie, I have to say that the liberties taken were surprisingly small. In fact I'm not sure I've ever seen any movie based on a novel try so hard to be faithful to the source material.Unfortunately there's a downside -- there were a number of scenes (too many of them) that seemed quite forced precisely because they were trying so hard to cram in themes from the book that there just wasn't time for in a two hour movie.
This is an age old dilemma; What do you want? A more faithful adaptation of the source material or a better movie?
As a fan of the novel, I have to say I am extremely pleased to see that this movie was made by people who understood and clearly cared about the novel. On the other hand, I think science fiction cinema really suffered for this choice. I'm really torn. I really think they should have taken more liberties with the story and made a better movie. On the other hand, if they'd done that I'd probably be writing a blog post complaining about the changes.
One last thing to think about -- Ender's Game was originally a short story before it was a novel. Adapting the short story might have made for a much better movie. But then you end up with a movie that doesn't address the morality of the xenocide against the Buggers at all, which is an important (and maybe the most important) theme of the novel.