Just watched the movie last night, and here's the list I could put together at this time:
There's a difference right from the start, and that is that there's been only one prior invasion from the Formics. No scorching of China, I guess.
Also in that scene of Mazer's victory, it's staggering to see that the ship he targetted was like 100 times bigger than the fighters, so it would obviously be a "command post". From the book, I expected the queen's ship to be completely identical to the other Formic fighters...
Of very little relevance is the fact that Stilson's name was not mentioned at all in the movie. Or did I miss a tag on his chest or something?
Also understandable is the lack of time devoted to explaining the population limitation laws, a context that would make having three children illegal unless under special circumstances.
The game that Peter played was called Formics and astronauts, not buggers and astronauts like in the book.
The shuttle launch had Bean in it; didn't allow for enough time to get Bernard's arm broken... And they actually went against the book's explicit "vomit in null gravity wouldn't be fun"!
Orientation/travel within the Battle School: only one guiding light, not a sequence of three coloured dots...
A singular message in the cyber-contest against Bernard was included, but it was too short and I couldn't spot if it was also signed "God".
Battle Room is a sphere instead of a cube. The winning condition doesn't require four team-mates to unlock the forcefield over the door.
The first freezings in the Battle Room should not have Ender being shot at (in the chest) by his partner!
Hand-to-hand combat is actually compulsory in the movie.
The change of leadership within the group differs from the book (or so I thought I saw) : from Bernard to Ender instead of to Alai.
The mind game is very much condensed. and the Giant's Drink is supposed to be two different liquids... And the "deaths" should be more diversified.
No other games are mentioned. At least we did get to see a glimpse of one right before the monitor scene in the beginning.
The Salamanders being the best, and undefeated for 21 games. Too hard to swallow, this...
The Salamander game is also a condensed version of events. Ender's place at the top of the standings because of his insubordination isn't emphasized.
No practice sessions with other launchies and "lesser" soldiers. No scuffle.
Not being part of the Rat army, or of Petra's Phoenix army. Petra staying in Bonzo's army until the last game.
As already mentioned in the question, no "politics on the nets" subplot.
No letter from Val as he gets stuck on the mind game...
Instead, there's an ever so slight hint of a romantic connection with Petra.
Bernard as part of the Dragon army (and later as part of Ender's jeesh)!
Dragon army ever being below the top position...
The Dragon vs Salamander + Leopard game is another condensed version of events. And still, some of the imbalances weren't pictured. (e.g., 2 games in one day, thawing of enemies after 5 minutes)
The rage against the teachers and the quitting after the final fight seem to arise from different reasons (and only after the battle with Bonzo)
Dap being at the Command School. Command School being on some planet somewhere in between the Earth and the Formic homeworld, instead of inside Eros.
Ender didn't retaliate after his first encounter with Mazer.
Petra doesn't fail during the final campaign... Actually, I think it was Dink who erred in the movie, and the consequence was a "failed mission", not a Pyrrhic victory.
The ships in the last battle aren't older than the rest of the ships in the campaign.
The Formics' home world is still structurally stable after the MD was used against it!!! (the only sensible outcome of such an attack would be as described in the book: "where the vast enemy fleet had been, and the planet they protected, there was nothing meaningful. A lump of dirt was growing as gravity drew much of the debris downward again. It was glowing hot and spinning visibly; it was also much smaller than the world had been before. Much of its mass was now a cloud still flowing outward.")
No League War after the end of the bugger war.
Anderson is a woman in the movie. I might have missed some clues in the book, but (until i'll examine more carefully) I think only the Epilogue asserts that he's a man.
No mention of the colonization effort and/or the repeal of the population limitation laws.
And Ender's discovery of the queen pupa is much too sudden, not the eight (plus fifty) years into the future that the book said. And to top it all, a live-though-dying mother-queen!
See 21.5 and 24.5. I had forgotten about these, but I just remembered, and added them now in their place above.
See 4.5. Credit should go to ike for noticing this.
And I'm sure there are others, but these were the only ones that I could notice while viewing the movie for the first time.