Complete Monster: In The Gods of Mars, the Big Bad, Issus, ancient ruler of the First Born Black Martians, has manipulated the Martian religion into a cult surrounding her. Making the First Born into a violent force that routinely kill and enslave others, Issus lures victims to her domain in the Valley of Dor where they are killed or made into her slaves. Many are forced to fight in the arenas to die for her entertainment while the women are made into her handmaidens who only last a year before she kills and eats them. Throwing John Carter and friends into the arena, Issus has a slave revolt massacred when John gets them to rise up against her. Out of spite, Issus later locks John's beloved princess Dejah Thoris and two other women who love him into a chamber that will seal off, leaving them nothing to eat but one another. With no care for her own people, Issus stands as among the worst Barsoom has to offer.
Ensemble Darkhorse: Dejah Thoris, despite her decreased focus in succeeding books, is one of the most recognizable characters in the Barsoom series. She has been heavily depicted in John Carter's fan arts, popular pin-up subject among artists like Frank Frazetta and is given expanded roles in Dynamite Entertainment's comic adaptation, even starring her own regular series and original miniseries as Dejah of Mars and Dejah Thoris and the Green Men of Mars.
The novels are surprisingly progressive for their time. In particular, the Barsoomian races aren't portrayed as weaker or pathetic compared with each other and the white human protagonist. Far from it in fact, as the black Martians are invariably described as honorable and very attractive, despite most of them being sky pirates. What's more, Red Martians are the result of interbreeding every race together for survival and adaptability, and are typically thought of as the most sympathetic.
The cannibalistic White Martians are even nastier than the Black Martians. Clearly inferior morally to the Red Martians, intellectually to the Black Martians and technologically to the Yellow Martians, the White Martians are easily the most unpleasant of the Barsoomian races. Those hybrid Red Martians (and even some of the "monstrous" Green Martians) are good guys and clearly very intelligent, adapting very quickly once not oppressed by the others. The original trilogy even ends with John Carter becoming the eponymous warlord and uniting all the varying races of Barsoom as equals — and this is clearly shown to be a good thing. The portrayal of women has been compared favorably in some respects to even some modern works.
It is also stated that the cross-breeding that resulted in the Red Martians was intentional - an attempt to breed a race that could adapt better to the changing environment.
In the first book at least, John Carter is disgusted at the Green Barsoomians for their warlike ways and violent tendencies, but it is taken for granted that his killing of dozens of generally innocent people is perfectly justifiable because he is doing it to rescue his One True Love. The closest he comes to acknowledging that maybe murder isn't the best way to solve all his problems is when he reflects that the guards were Worthy Opponents and fighting men like him. An Alternate Character Interpretation might be that since the story is told in the first person, the reason John is portrayed as chaotic good despite his somewhat chaotic neutral method of interacting with obstacles is because he's actually a narcissistic sociopath who doesn't see anything wrong with sacrificing other people's lives if they get in the way of his personal happiness and that of those he cares about.
John is a former Confederate soldier. note At the time, the Confederacy was often seen as the more romantic side of the Civil War; as attitudes shifted, particularly about race, they became seen as a much more villainous side. Where once a Confederate hero might be expected, to modern readers it carries a whole bucket of Unfortunate Implications. The Film of the Book largely gets around the Unfortunate Implications by making it plain Carter was extremely disillusioned by the Civil War and felt little loyalty towards the Confederate cause
This is actually lampshaded in The Gods of Mars when Carter meets Black Martians for the first time; the narration says he finds them admirable fighters and that their skin tone only makes them more handsome, adding that the reader may find this 'odd for a Virginian to say'.
Fanon Discontinuity: John Carter and the Giant of Mars tends to fall victim to this. One notable example: in the book "A Guide to Barsoom" writer John Flint Roy clearly states he does not consider this story to be a true Barsoom story, and thus didn't include any information about this story and the characters apearing in it in his guide.
Harsher in Hindsight: Two decades before the Nazis rose to power, this series was giving us white, blonde-haired, blue-eyed people who considered themselves superior to all others.
In Gods of Mars and Warlord of Mars, there are suggestions of this between Dejah Thoris of Helium and Thuvia of Ptarth, who are imprisoned together with the villanous Phaidor in the Temple of the Sun. When Dejah Thoris is reunited with her beloved John Carter at the end of Warlord, she recounts how Thuvia's "tender love" kept her sane through the months of their imprisonment.
John Carter admires Xodar and calls him handsome and beautiful at certain points.
This runs in the family, as Tara of Helium and her devoted slave girlkiss in one scene - after the two get into a passionate argument about Tara's safety that includes them declaring their love for each other. While a certain amount of Values Dissonance applies to the interpretation, damn.
Moe: Sola of the Green Martians. She's kind, comforts Dejah when she's captured, cares for her father, and suffers a tragic backstory.
John Carter was preceded by an obscure book titled Lieutenant Gullivar Jones: His Vacation, which was about an Earthman transported to Mars through mystical means, fights aliens with his superhuman strength and falls in love with an alien princess. The book was published a decade before Burroughs wrote A Princess of Mars. Interestingly, both works crossed over in comic book format in Dynamite's Warriors of Mars (as well as the famously All Myths Are True comic series, The League of Extraordinary Gentlemen.)
Green Martians were orcs before it was cool. Hell, they even embodied aspects of both Tolkien and Blizzard orcs, being depicted as vicious and warlike in general, but some of their members were honorable to a fault - before these tropes were established.
The White Martians resemble the Nazis as supremacists with Aryan features such as blonde hair, fair skin and blue eyes, yet they predated the Nazis rise of power by decades.
Older Than They Think: Some fans of a little James Cameron film saw the trailer for the Disney adaptation and declared it a rip-off. Unknown to them, not only is this series older, but James Cameron himself has cited it as direct inspiration for his film.
Seasonal Rot: John Carter and the Giant of Mars is widely considered by fans to be the weakest of the Barsoom stories, and is generally considered something of an afterthought rather than a true John Carter novel. The fact that it wasn't written by Edgar Rice Burroughs himself but rather by his son John "Jack" Coleman Burroughs doesn't help either.
Seinfeld Is Unfunny: Probably at the core of a lot of the criticism the film version is receiving.
Values Dissonance: Burroughs was writing at a time when scientific racialism was in vogue, and it shows; the Red Martians, the most populous of the humanoid Martians, are said to have originated out of miscegenation between the White, Yellow, and Black Martians, who have been forced into polar enclaves by dysgenic pressure. The fact that the Black Martians are cannibalistic raiders is questionable in modern eyes.