And regarding the oxygen masks, at the end, when jake’s trying to get the mask, its written “Emergency Rebreather something” . “Rebreather” is the key word. As you said its something that takes pandora’s atmosphere and converts into human breathable form (We’re never told whether it has oxygen or not. Probably it does) .

So, so much for scientific dissection. I think most people are missing its magic !

//The flux vortex is a phenomenon on Pandora that occurs most strongly near the Tree of Souls, although it spreads through the Hallelujah Mountains. It is caused by large deposits of unobtainium, which create strong magnetic fields. This is what causes the Hallelujah Mountains to float. The vortex also causes disturbances in aircraft instruments, rendering them unreadable. It also prevents guided missiles from working properly// says

http://james-camerons
avatar.wikia.com/wiki/Flux_Vortex

Google for more..

Reg..POWs…yep I was wrong..they are wearing Masks…Oops!

On The scaling glitch betwwen humans and Na’vi:
Did you doze off in the scene when sammy worthington is mapped into his Na’vi avatar??
Everything was upto scale and satisfactory….
And mind you there was that scene in which the ‘Tsahik’ tries to transfer Weaver’s soul from her human body to her Avatar..

Dude its 150 years into the future so u may never know about the advances in nanotechnology…
There could be a gas filter using nanotechnology that would feed the the oxygen into the mask from pandora’s atmosphere..(just speculating..:D)
In 1820 no one would even dreamed about going to moon.
Watch the movie again all the pow’s wear gas mask at the end.

Well about the story part i agree its quite obvious but the screenplay came to its rescue.i didnt even blink an eye through the movie.

Well about sammy switchin sides:

They are only debriefed that they would be goin into pandora to learn about it and hopefully talk their way out and convince the people to vacate the hometree.
For jake, he is a handicap and he is more agile, athletic and more importantly walk and run in his na’vi avatar so its natural that he is a bit reluctant to give up his na’vi identity, but then cumon dude he is the HERO of this movie and obviously he is good hearted and destroys the evil force by joining hands with the good force..

** Story is VERY Ordinary..
** Its impossible to believe Worthington and gang will go all against and destroy their fellow human fellas…coz…they are coming to pandora by knowing the mission…(worthington may be an exception as he had sex with Neitiri )
** Human/Na’vis/Creatures/their livelihood are freely walking/existing in Pandora like they do in the Earth….means gravity (or acceleration due to gravity) is there in Pandora..the giant water falls is evident for the gravity….but Cameron boss how come the flying mountains..?
** Human/Navi size proportions are not differentiated properly through out the movie.Only in the initial scene where the Navis had been cloned, the size difference was evident…
** The oxygen mask for humans are just for the name sake..where are the cylinders?..alas in the closing scene the POWs are shown without masks…

These are all only few…as they say in Tamil…Water has froth,Rice has its shell so before consuming them remove the unwanted and be happy…

But still in my opinion instead of going for the animation for the Navis(Navis only) Cameron would have gone for the human artists with special make-ups…then the movie would have looked more real and life.

Cheers!! and Happy New Year!!!