Bookender's Game

First principles first. A novel is not a movie; a movie is not a novel. Each medium has its own formal logic. A screenplay adaptation of a novel is, necessarily, a translation. So, a decent adaptation distills the essence of the book, though changing details (or visualizing implied details the novel didn't spell out).

So, Stanley Kubrick's A Clockwork Orange was a great adaptation. Kubrick's The Shining was not. Why? To paraphrase Faulkner, a great story reveals the human heart in conflict with itself. Filmwise, A Clockwork Orange examined the stunted free will of the sociopathic Alex. The Shining reduced Jack Torrence to a scary monster with an axe. One flick brought the heart of the novel to the screen; the other stabbed the novel in the heart. So it goes.

That said, here's my take on Ender's Game, the movie. (Spoilers ahead, kids!) It's faithful to Orson Scott Card's book, though it airbrushes some details. Specifically ...

Say "what." Say "what" again.
Ansible. Third. The movie tosses off such terms without explanation. If you've read the book, you know that the Ansible is an FTL communications device reverse-engineered from Formic tech; a "third" is a despised third child in an overpopulated world where more than three offspring are illegal -- and only allowed in rare exceptions like, say, breeding a super warrior to battle ants in space. A few insane geniuses in the audience might suss this out from the flick, employing deductive reasoning and suchlike. The vast majority will be clueless.

Faster! Faster would be better!
The movie compresses scenes. A lot. Ender's on earth, Ender's in battle school, Ender destroys the Formics. That's all, folks. 

Ender game.
Ender lives up to his name. A nice kid, but if you !@# with him, he ends you. In the book, he clearly snips the life threads of Stilson and Bonzo. In the film he just kinda hurts 'em. Puts 'em in the hospital. They may, in fact, die later. But it's off-screen.

Psycho brother, qu'est que c'est?
In the novel, Ender's bro, Peter, is a stone-cold, squirrel-torturing, cold-ass, dead-eyed, sociopathic, psychopathic sack of damaged goods. In the movie, he's just kinda mean.

Demosthenes, Locke, stock and barrel
In the novel, Ender's siblings, Peter and Valentine -- anticipating the postwar resumption of global tensions -- become Internet pontificators on a global scale, under the pseudonyms of Locke and Demosthenes. (He, the conservative, she the liberal.) This subplot plays out in subsequent novels in the Ender series. In the flick, this subplot is entirely lost.

Ecce LGBT
Evidently, there were no gay people in The Book of Mormon. (The actual book, not the Broadway smash.) Orson Scott Card, taking said book at face value, has spoken out against gay marriage and such; the gay community has responded in kind with calls to boycott the flick. The poor, damn producers have, wisely, ducked the issue. Thus, the flick is nice and sparkling clean of any taint of homophobia, O my brothers. In the original novel, "buggers" was the slang term for the insectoid space enemy. Not in the movie. Novel Ender insulted Bernard by posting, "Cover your butt. Bernard is watching. - God." Film Ender's insult: "Bernard is living proof that they still send chimps into space." Such snips don't neuter the movie. The big damn tragedy: Ender's Game is, ultimately, an antiwar, anti-bullying plea for tolerance. Ironic, ain't it?

That's all, folks
The novel wraps up with a complicated, nuanced coda evoking the Formics' awareness of Ender, via the Ansible, Ender's subsequent quest to find a home for the last remaining Formic egg, all in the context of Valentine and Peter's hegemony and the time dilation effect. In the movie, the last surviving Queen wipes away Ender's tears, gives him an egg, and that's it.

Conclusion
As a devoted writer of unproduced screenplays, I know the flop-sweat inducing pressure of all those 14-year old bastards out there. Simplify! Simplify! The scene is going on too long. The audience is BORED! So, I dig the need to make the movie move. There's material enough for a mini-series in the first book alone. The movie clocked in at 1 hour and 45 minutes. Fans of the book will mourn the cuts; the unread won't. Verdict? The movie is great; another 15 minutes would've made it better. The cuts went a little too deep. I enjoyed it, yeah. I understand. But an intelligent audience is the only audience this flick could hope for. Dumbing it down -- even a little -- was not the right move.

Thanks & a hattip to Andrew Fugate for metatextual insights.