So much was absent from the book. Some of the things they did include from the book made the absent things conspicuous because of sheer weirdness. Like the Mind-Game that Ender played during his downtime. It was so strange in the book that I'm kinda surprised they put as much of it in the movie as they did.
They really glossed over Battle School. It was in battle School that the reader was shown that Ender was a genius. In the movie, despite being and entirely visual medium, we're simply told over and over that he's a genius.
I kinda missed the B-plot involving Valentine and Peter back on Earth, but ultimately, it was never truly vital to the story. It would have been cool to see that Peter was like Dexter in the future and how vital Val was as a balancing force between Peter and Ender.
But whatever. It was okay.
"You must be proud, bold, pleasant, resolute,
And now and then stab, as occasion serves."
Edward II: Act 2 Scene 1, by Christopher Marlowe