Look, the book is silly. But there were hints, little bits, that showed that the movie could have been so, so much better. But let's start with characterization.
Book!Carter is a man who is proud to have done his duty, and is happy to go wherever he has to in order to carve out a life for himself. While it's subtle, we have to remember that we're looking at a man who fought for a cause that was destroyed. His world was completely altered by the war. His arrival on Mars gave him a new cause to fight for, and he was glad of it.
Disney took this, gave his previous occupation a passing mention, and made him the stereotype of an Angry Ex Soldier. He's not in this for your revolution, Princess!
Speaking of stereotypes, let's talk about Dejah Thoris. Look, she wasn't the best character in the book. She was mainly a Dulcinea for poor Carter, but at the very least there was mutual respect between them, and they actually had time to develop a relationship. Dejah, whatever her weakness, was characterized as a brave woman who was willing to do whatever it took to save her people-including marry the guy she did not want to marry. (For the record, Sab Than was mainly just this decent guy who was in love with her, and his dad was the jerk. But he had to die, so Carter could boink her instead. Poor Sab Than.)
Disney decided to make Dejah a Strong Female Character. She is a warrior and a scientist. And I have no problem with that. One of the better moments in the movie is when Carter tries to be gentlemanly only to find she can hold her own in a fight. Her being the scientist to discover how to save her planet is also keeping with her characterization of someone determined to save her home.
The problem is that these good added traits are undermined by turning Dejah, not into a strong character, but a Strong Female Character. This means, basically, that she throws temper tantrums when things do not go her way. Unlike her calm, wise book counterpart, she flips out at her father, consistently lies to Carter, and finally screams, "I CANNOT MARRY HIM!" in the manner of every ridiculous novel heroine ever. Book Dejah may not have fought, but compare her to the petulant child in the movie and she comes out looking much better. Why do we think that strong characters have to scream and rage and wail? Why do we think that emotion can only be conveyed in this way? I can't see Dejah from the movie standing with her back straight and head held high when facing torture. I can see her screaming and flailing uselessly though, and it doesn't look quite so good.
This also leads me to wonder, what is it with movies and the way they characterize relationships? In the book, yes, Carter is immediately in love with Dejah. But he spends time actually getting to know her long before they get together. His initial attraction is built upon by mutual respect, and even though Dejah can be supercilious at times, you can see why he becomes so attached to her. In the movie, they fight and argue and continually distrust each other, all whilst smoldering at one another. Apparently this is a good enough basis for a marriage after, what, a week at most? They barely trusted one another up until almost the very end.
Let's talk about Carter's other relationships in this movie. The only one that stayed really true to the book is his friendship with Tars Tarkas. And, by the way, that's one part I love. Tars Tarkas and his goofy grin. No, that grin was goofy, and it was hilarious.
But then, we get to Sola. Sola's part was severely cut in the movie, and it's sad. One of the best parts of the book is her growing friendship with Carter, and her helping him and Dejah because her own parents were truly in love, and she knows the value of it. In the movie, she is basically reduced to a third wheel that helplessly tells Carter and Dejah to stop doing unwise things while being ignored. Her entire backstory of being raised by her mother in secret for several years is barely mentioned, and instead of coming from her, Carter randomly guesses it. Her story isn't intertwined with Carter's because she wants to help him do what's right. It's intertwined with Carter's because Carter is the hero and we say so. Her one moment to shine is quickly eclipsed by Carter Being Awesome.
In terms of story, it was clunky, and with little pacing. The Therns confused the heck out of me, because I haven't read any of the other books yet, so to me they were just a bunch of annoying Self-Righteous Higher Life Forms making specious arguments for why they're The Best Beings Ever while they show off their Dr. Manhattan eyes and manipulating events that don't even need to be manipulated.
Also, everyone has midriff baring armor. Why? I imagine the discussion went something like this:
What did I like? Tars Tarkas' goofy grin. The design of the green martians in general. Kantos Kan being more attractive and engaging than the hero in, like, one scene. (Okay, Carter was attractive until they shaved him and made him look like he hadn't quite gone through puberty yet.) Woola. Woola was awesome. Edgar Rice Burroughs looking as confused as the audience. Virginia being shouted with joy by millions.
It's just a shame that they didn't do the story or the characters justice.