According to Wikipedia
The film is largely based on A Princess of Mars, the first novel to feature the hero John Carter.
What are the plot differences between the two of them?
He didn't learn the Barsoomian language via a potion, he learned over the period of a couple of days.
He died on earth and was magically transported to Mars. There was never a Talisman.
There was no super weapon.
Woola was a faithful companion, but wasn't that fast.
He could read Martian minds in the book; I think that is how he knew the Chief had a secret daughter and the daughter never knew her dad for her safety.
Dejah Thoris, was unable to marry John Carter, because she had accepted the bad Jeddak's proposal and she could not go back on her word even if John Carter killed the bad Jeddak.
There were no shape shifter super bad guys that I can remember.
If I may add to the all correct DannyBGood points:
1. Zodanga wasn't a walking city
2. Dejah wasn't a "professor" (nor was she an archeologist)
3. Iss temple wasn't a high-tech planetarium
4. John Carter wasn't just a clumsy jumper who in the end got angry (they say in the first scenes he is a great soldier, apparently the humour of Disney-Barsoom causes him to forget all about war strategy and ethics)
Despite all these inaccuracies, I enjoyed the movie. Don't forget Giacchino.
The first ten minutes of the movie were posted online somewhere. After watching that clip and reading the beginning of the book, the most obvious difference was in the character of John Carter himself.
In the book, he was a traditional Southern Gentleman - well educated, well spoken, and well mannered.
In the movie, he was an unkempt, drunken, disgruntled thug.
In the book, John Carter is of mysterious background. He doesn't remember his childhood, doesn't age, and only remembers being a warrior. The implication being that he's immortal. The Burroughs character is not a true nephew, but rather John Carter is a close friend of his family as a child.
Carter travels to Mars via astral projection. He actually walks around on Earth briefly and knows he is going to Mars as he is gazing at it and flies toward it at incredible speed.
The Tharks have no sense of compassion whatsoever. They find humor in violence, even of their own kind. The Red Men are not much better. They do not kill their own eggs after the hatching. They are a little larger than depicted in the film, and are hugely muscular - including the women, not the skinny things depicted in the movie. Tars Tarkus is a mighty warrior chief (a Jed), not a weakling dependent on out-thinking his opponents, but he is very intelligent as well. He is one of many minor chiefs and there is a capital city, several days journey away, to which he brings Carter to participate in the arena - which is ruled by a fat lazy and vicious king (Jeddak) The "milk" is from a plant and becomes Carter's chief source of nutrition and water. The Therns are bald but they wear yellow wigs in shame. They don't make an appearance at all in the first book and there is not visit to the Gates of Iss in the first book. When Carter does go in the second book, he finds the White Men (Therns) are not supernatural (as is believed by the Red and Green men) or technologically advanced, but that they are religious fanatics deluded into thinking themselves gods. The Temple of Issus (in the second book) is inhabited by the Black men of Mars (but not black in the same sense of the Earth races, their skin is metallic or stone-like, and reflective). There's pieces of the first two books scatter. Kantos Kan (played by James Purefoy) is in the second book. There's a crisis at the end of the first book to save Mars that is not in the movie, but some hints to it are, such as the racing little flier.
My favorite element of the book however is the florid language. It's beautiful how they speak, almost poetic. Other movies have embraced the creative use of speech with success. It could never be copied exactly (in some instances the sheer quantity of words spoken doesn't fit whatsoever with the fighting taking place simultaneously.. like a half of a page of intense intellectual conversation during just a few sword strokes..) but at least a nod would be fitting.
Barsoomians have the capability to remote view other planets and are familiar with Earth's inhabitants and culture.
And there's the matter of everyone on Mars being completely naked all the time
They had to take a lot of liberties to turn this story into a good movie. The characters had absolutely no depth in any of the books. It was good guys vs bad guys and one chick that kept going back and forth between evil and good and you couldn't be sure about her. But the good guys were pretty much all like all the other good guys and the bad guys were all pretty much all the same too. All the ladies were there to look pretty and be rescued an they all immediately fell in love with JC. They really had to stretch to make the characters interesting. I guess that is why they decided to give the Therns some super tech to spice it up and give JC a strong villain he otherwise would not have. Also, they had to make the movie more PG and PC. In the books everyone was basically naked except in cold climates. Dejah is portrayed as some irresistible sex goddess no man can resist. They had to tone that stuff down, there is a lot of sexist stuff and racist stuff in these books because they were written around WWI time. Finally you have to do some stuff, like drink a potion to know the Barsoomian language in order to save time. This an expensive movie. Time matters.