- Battle school, as others mentioned, is waaay too short. You really only see (or even hear about) two battles; Ender's first and Ender's last. Right before he's about to be promoted out of Battle School, you don't even know how Ender and his army is doing. The only time they even mention his success in battle school is when Graff says 'you're at the top of your grade in scores, and your army is at the top rank'... and a viewer is just kind of like "oh, cool, guess he did well." Should have fit in at least one more battle, showing off his talent and developing relationships with his team further (and give them time to shine; LOOKING AT YOU BEAN).
- Bean is just another guy, not really anything/one special. Same goes for his whole team really; the relationships just weren't developed very well aside from Petra (which yeah, is some weird almost-love-story between adolescent kids).
- Command school, like Battle School, is only really two battles; first and last, with a very quick run-through of other simulations fought during a narrative (battle school could've benefit from this!). I could've gone either way with this, but ultimately another battle could've helped again with developing the characters and an appreciation for Ender's talent.
- Locke/Demosthenes are 100% non-existent, but honestly I'm OK with this, as there was no way they could really fit this part in the movie.
- Probably my biggest problem: You never hear anything about this war from the bugger's viewpoint. Ender does ultimately find the queen's egg, but there is no communication/mutual understanding of any sort (unlike in the book), and you don't feel much real sympathy for the buggers. The book makes you feel sympathetic, and even kind of makes it an ironic tragedy... the movie does not have this. The movie makes it seem like Ender just feels bad and wants to clear his conscience in finding a world for the egg, which while true to an extent, isn't the only thing at play here. The viewer will likely leave the movie thinking they're a dumb species that f'd with the wrong people and got rightfully curb-stomped, but managed to mind-manipulate Ender into feeling bad about it right before perishing.
- Ansible is never explained, at all. It's brought up only twice: 'we communicate to our fleet by ansible' and 'the ansible network is down'... and nothing further is ever said about it. Ender never even questions it. Someone who hasn't read the book would just be like "uh, oh, OK."
- Minor: Seems kind of silly to show us fighting the first bugger battle with F14 jets, and then you see the advanced space armada we launch towards their home world ~20y later. Viewers likely to be left confused on this and other details surrounding our tech.
- I know it's typical that a movie appears rushed compared to a book, but this was pretty excessive. The dialogue was seriously lacking on some parts. Ender & Valentine on the lake was literally not much more than:
Valentine: They want me to convince you to go back.
Ender: I won't go back.
Valentine: But you have to for me.
Ender: OK, I'll miss you.
Spanned over [i]maybe[i] one whole minute. Not that I really needed that particular part to be longer, but found it to be an amusing example.
It also made for a lot of awkward dialogue. There must've been a pretty big push to get a lot of book quotes in, because they're forced in at so many points that don't necessarily make sense, and seem more to be like "we're touching on an entire theme/chapter/storyline of the book with a one-liner, just for the readers!"... while making it super awkward/forced for a viewer.