Where to start?
Jake starts off as a ******* grunt, with no appreciation of the land, spirituality, etc.
Neteri is an enlightened Na'vi who teaches him the beauty of the land, spirituality, etc, etc.
By the end of A1, they've all grown into different better/evolved/enlightened characters, with a real appreciation for what they have, etc.
Jake unites all the Na'vi - even a ocean tribe - and they save their planet from the greedy humans.
A2
Jake is just a dumb grunt again, primarily using guns, with the brains of a peanut, and yelling at his idiotic kids the whole time, without any care nor consideration for the tribe he take his idiotic family to.
Neteri just whines, cries, or screams throughout number 2 and has lost that enlightenment to become nothing more than a stepford mom.
Jake is so intelligent that rather than just reuniting the tribes again to ward off the far smaller threat of skypeople he take his woes to some other tribe - genius! - even though he knows the sky people will come for him.
They couldn't come up with a new villain so let's do the laziest thing and bring back the old one in the most stupid way possible.
A 70 year old actor (Weaver) trying - poorly - to inmate a teenager.
A ******* human who conveniently is the son of the main antagonist.
A ******* Aussie actor who couldn't act his way out of a paper bag playing some inane character.
A completely unnecessary new highly sort-after resource.
There's so many lazy decisions that make no sense, all for the sake of spectacle, that I could rant for another three pages.
If **** for brains Cameron wanted the ocean tribe to be the main tribe (just so he could show off his love of the ocean), they could have easily just had the ocean tribe from the first film reach out and ask for help because the humans were trying to take their land this time. Would have been far more organic and natural than idiot Jake being so selfishly inattentive.
Number 1 is a solid film that has a clear logical narrative, where all that is foreshadowed in answered perfectly.
2 is just a piss-poor, cash-grab, excuse of a script (as will all the sequels be), that made no sense and didn't organically expand on the foundations in 1. Pretty lights, no substance, and no subtext... and no brain cells required. Switch off, with your mouth agape, and let your senses be stimulated like an aimless roller coaster ride.
EDIT: think how brilliantly he handled Aliens - that's how to organically expand on an original story with integrity, intelligence and class.
