For once, I don't have one of my lists for you to read, cause while there are distinct differences from the book to the movie, there is no need to list them. Instead, I'm going to simply talk about what I liked.
I liked the reason the movie gave for Mars needing mom's in the first place. It wasn't just about soccer practice and cooking. It was about discipline. They need moms who could set rules so they could steal their abilities.
I liked the family aspect, which was what the story was about. The relationship of Milo and his mom. They movie kept that including the scene in the end where the mom gives him her helmet up so Milo can breath and Milo in returns gets a helmet for her. The addition the movie made was the family dynamic of mars itself, how boys were thrown to the wasteland thus eliminating the family structure that existed decades before. And, in the end, the women wanted that structure back. They wanted their families.
Last, I liked the addition of Gribble. This allowed Milo to have some company on the planet, someone who understands his need to save his mother, someone who was in his shoes not so long ago. Gribble was the comic relief character and he was good at it.