These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
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.
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.
One-Scene Wonder: Solan. The guy appears very briefly, but he gives Carter a better fight than anyone else in the series.
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: So, so much. Burroughs was writing at a time when scientific racialism was in vogue, and it shows; the Red Martians, the most populous (and least advanced technologically) 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...
Fair for Its Day: ...however, this is clearly subverted because the cannibalistic White Martians are even nastier. 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 many 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's even possible that the whole setup was a Take That to the racism of the era.
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.
Also, he 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.