Reading a book like A Princess of Mars you have to make certain allowances.
The story is almost 100 years old and pre-dates science fiction as an established genre and it was written in a pulp style. Inevitably the tone and references are dated. Additionally some of the elements which would have seemed bold and original at the time now appear heavily over-used.
It might be tempting to rate the book more highly for its huge significance and influence on the sci-fi genre, but that is really a separate issue to how enjoyable the story is itself.
It's certainly an easy read, moving quickly from adventure to adventure and keeping its hero in peril. But even the quick pace can't disguise the flaws.
There are just a few too many coincidences. A few too many times when John Carter is able to casually get himself into just exactly the right location or have just exactly the right conversation. And those story flaws are hard for me to ignore.
So in the end I come away from the book dissatisfied. I see the elements of what made me love things like Flash Gordon here. I recognize it's significance, that it pre-dates so many of the things I like.
But in the end I couldn't connect with the "perfection" that is John Carter and while the world is described in considerable detail, I found the characters largely one dimensional and just could not connect with it the way I had hoped.