The main things that stuck in my head were the unnecessary changes to the book that opens up holes.
Why wasn't Command School on Eros? I suppose you can cut them slack because they wouldn't be able to go the ending plotline otherwise (which was stupid IMO)
Why condense the first and second Invasions?
The first invasion was only one ship that scoured China.
The second invasion (With Mazer) was something more similar to what happened in the film, except Mazer wasn't a fighter pilot and it was set in the asteroid belt. The original story was an scouring ship to cleanse the planet of current life and start introducing Formic life (with current technology), the second invasion was 50 years later, after humanity learned from captured Formic technology and ramped up for interstellar war, and the events of the film curred around 70 years after that....
And most importantly.....why change the tactics of the final battle?
"Okay, first I am going to show you what I am going to do....and now I am going to do it to your planet...."
I laughed when Indiana Jones called it brilliant.
IMO, the biggest casualty to the story telling (which is due to the medium rather than any creative flaw), is the fact that they weren't able to portray the slow wearing down of Ender.
And they detracted from the whole damn point by not killing Bonzo and Stilson, and making the Bonzo attack more of an accidental result than Ender's killer instinct that he later shows remorse over.
This post was edited on 11/4 at 6:54 pm