I read my first Burroughs novel, Tarzan of the Apes, when I was in 5th grade. I burned through all 20 something of them. Then, finally, B. Dalton Bookstore in Cortana carried the John Carter series. I begged my mom until she bought all of them for me at one time, and I devoured them. This was way back before the internet, and getting my hands on these was like finding gold. Needless to say, I'm a Burroughs nerd. Sure, his style becomes formulaic and predictable, but his imagination was untouchable. For a middle school kid, it's easy to look past the plot contrivances and holes when fantastic worlds are being created in front of you.
I have been waiting for a John Carter movie a long time. When it finally came out, I had read the mixed reviews and was aware of a lot of the hurdles that the movie faced. I saw it on opening day. Sure, it's got some weaknesses, but it still was an awesome ride, and did the novels justice for the most part.
Here's some answers to your questions:
Watched it last night. They did a poor job with the story. Nothing made sense.
Even the fact that he could jump great distances, kill a whatevertheywere with one punch, yet could get knocked down easily was confusing. - Mars is slightly smaller than earth, thus it has a weaker gravity. John Carter's earthborn physiology made him a virtual "superman" on Mars as far as strength to weight ratio. He can leap far distances, and is super strong compared to the denizens of Barsoom. But he's not invulnerable, he can still get hurt, knocked down etc.
Why were there different green people? - Barsoom has different races on it. The Red Men look like humans, the Therns (the bad guys in this) are more white but human like as well, the Green men are a nomadic warrior race called Tharks.
How could a civilization that had flight not know about a huge temple/machine thing surrounded by a river, especially if that world has almost no water? - The civilizations of Mars are MUCH older than earth and the planet is dying. The seas have dried up and countless wars have decimated most of the inhabitants. Roaming bands of Tharks shoot down explorers so most cities are isolated. A lot has been lost. The river is called The River Iss and is similar to the River Styx, it is the river one takes to death. No one comes back from it or tells tales of it.
The novels are worth the read. Just suspend some cynicism and enjoy them for what they are, the FIRST dive in to what would become science fiction and fantasy. 5 Movies that wouldn't exist if not for John Carter