Alright I'm going to type this from my phone so it won't be perfect.
Ender's game as its own movie wasn't super terrible, though it definitely wasn't good. The problem was what they did to the book it was based on. After seeing what Hollywood has done with lotr, harry potter, and even the hunger games, enders game was an incredibly disappointing adaptation of one of the most popular SciFi books ever.
The first real problem with the movie was the pacing of the movie. Everything went way too fast, and you never got to see the characters develop, or realize what a sacrifice they made. In the movie, ender and his fellow students don't age. The whole thing takes place in a few months, maybe a year tops. The book starts when ender is six years old and ends when he's 12. He gives up his entire life to save the world. This squishing of the timeline males lots of the story seem much less dramatic than it is. In the movie when ender goes back to earth to see valentine, he's been gone for 4 years. When he sees her in the movie, its like he's been away at summer camp, and it makes him seem like a kid who's just homesick, rather than a hero who's given his life to save his planet and is *this close* to giving up.
The next problem was casting. My two biggest problems with casting were the actor who played bonzo, and the abundance of female actors. Gonzo is supposed to be 4 years older than ender, and is described as towering over him, cause he's 13 and ender is 9. In the movie, he's a twerp. Sure he's more built than ender is, but it doesn't look like the entirely 1 sided David and Goliath fight it was meant to be. So when ender fights him in the shower, ender has failed to do what the fight with gonzo was supposed to do: prove that he can win an unwinnable battle. The teachers knew that gonzo was dangerous to ender, but to them he was only valuable if he could beat an opponent who was stronger than him in every way. Since gonzo looked so puny, it simply made the teachers look like incompetents. An aside about the fight: ender should not have killed gonzo by accidents as was portrayed in the movie. The point of enders character is that he fights to win. That's what makes him the hero earth needs. They need someone to end the bugger threat forever. Mazer Rackham got lucky when he defeated the buggers. Earth couldn't rely on luck again.
Now the abundance of women: I understand that Hollywood has to be PC and make the movie appeal to little girls as well, but making half of battle school, and major Anderson, women was a big mistake. The primary reason is that by making so many characters women, petras character is diminished. Petra is supposed to be one of four girls in the entire battle school at that time. Her success in a male dominated world (because the military is undeniably male) proves she's exceptional. She's an outsider, which is why she helps ender when he joins salamander army. She needs him as much as he needs her. With so many women in battle schOol, we never get the idea that petra really stands out. Also the mild romance they alluded to between petra and ender was completely fabricated Hollywood nonsense.
The last major problem was scale. The armies were too small, the school was too small, they pumped too much of the budget into special effects and as a result, battle school came out looking like summer camp. The armies were meant to be 40 soldiers, and I counted 15.
These issues, along with all the little things, such as bean being in enders launch group, completely ignoring bean as a character, ended never joining rat army, on and on... made the movie a huge disappointment to fans of the book. As a standalone movie? 5/10. As Ender's Game? 2/10.