This is the movie Avatar should have been (yes, Avatar was bad). Here you have a whole world properly imagined, without any of the noble savage baggage or Fern Gully hippy BS that weighted down Avatar. Yet it still has a message, and it still delivers entertainment just like its pulpy forbears did.
When a director previously known for WALL-E makes a movie like this, you don't just sit through the whole thing without thinking about the message, no matter how entertaining (or not) you find it. John Carter does have a message that builds on WALL-E's - human civilization is on a crash course for destroying the environment of its planet. Andrew Stanton smartly expands on this simple message, fleshing it out by using a very old science fiction universe to move the drama of such a conflict over to Barsoom, another planet (very like Mars back in the days when we thought ancient alien canals ran over its surfaces due to defects in our telescopes), where another race of men are experiencing the end results of rampant expansion and civilization at the expense of the environment, resulting in the barren desert landscape that Mars is known for.
This is a brilliant move for Stanton to take because it builds on a theme that was actually already present in the original novels of Barsoom, generates a cultural cachet with the older generation, and uses it to comment on a very modern problem - our global climate change crisis. Whereas WALL-E was for children (wonderfully), and it left the reasons for Earth's destruction simply as gluttony and waste, here the reasons get a little bit more sophisticated - Barsoom and Earth are both seen as doomed by the demigod-like Therns, who seek to manage what they believe is the inevitable downfall of any civilization so that it does not disrupt their control of events even as the planetary ecosystem crumbles to dust around them. A more damning sketch of climate change deniers could not be painted. Stanton makes the humans (the ones responsible for the destruction of the ecosystem) almost unwitting players in this planetary-scale tragedy, blind to advances in science that could save them because of old beliefs in fatalism and internecine conflicts they cannot forget. Nobody is bad, as Avatar would have us facilely believe - instead they are only human (or Thark, as the case may be). John Carter (played by a barely capable Taylor Kitsch) has the job of trying to convince the viewer through his superhuman actions that we can transcend these inherent weaknesses if we can leap no matter how insurmountable the odds.
This message is subtly established over the course of a plot full of properly swashbuckling heroics, including naval combat, pitched battles, gladiatorial forays, journeys into forbidden realms, super-intelligent damsels in distress, geopolitics, strange cultures, and of course, the Western-sky beauty of that dying planet Barsoom herself. That most viewers didn't grasp the message consciously can be forgiven, since they had this to preoccupy themselves with. This is to say nothing of the good visual jokes and sometimes even funny dialogue, although at times the script does fall over itself, especially with a few bad lines - but these only serve to enhance the pulp atmosphere if you're of the right mindset.
Second viewings for the sake of watching a "bad movie" will surprise viewers with the revelation of a deeper purpose to all the sound and fury on-screen, but of course most will sadly write this film off and never give it a second chance.
One last final note about the Tharks - they are the most even-handed, complex, and believable alien races I've ever seen on-screen. SF movie-makers take note: THIS is how you create a tribal alien race without blatantly ripping off a stereotype of a random culture on Earth. They are barely anthropoid, have bizarre, sometimes superstitious rituals that would seem morally repugnant to a human, and have many faults (including a hopefully cultural propensity to violence). But they are also fair, honorable, strong, brave, funny, and open to change. Even better, all of this is subtly laid out over the course of developing the greater plot, without ever wasting any time on dreary and unlikely explanations, as Avatar did.
Instead of explaining itself, John Carter asks you to accept what you do not understand, to imagine it as you will while it concerns itself with the more important task of telling you a thrilling story that can inform your beliefs, if you let it.