Well - just finished watching it and I can only conclude that this film is for juveniles
In the first film - there were issues, to be sure. The premise of the film was to mine unobtanium. What happened to nuclear power? What about solar energy? The reasons for the energy crisis are unknownium. At least, you had scientists in that film - and humanity wasn't depicted as completely dumb.
In this film, any pretense of humans comprising of diverse people with varying intelligence is gone - they've all gone pure evil and dumb - it is so in your face, it's laughable and makes the first film seem like a masterpiece (even though that film had similar issues as well on a much smaller scale).
Because the writers lacked any sort of imagination, old villains make a come back, being resurrected from the dead. Speaking of which - considering that one of the secondary reasons for returning to this planet was to obtain the whale brain enzymes which grand humans immortality, one has to wonder - doesn't the ability to download/store memories and upload them into new bodies (Na'vi bodies, in this case) already solve the immortality issue?
There are all sort of events where characters act oddly and out of character. At the same time, in some segments, even bigger oddities occur. During the big conflict, Na'vi attack the ship to save the kids - yet.... at a certain point, all of the water Na'vi disappear, leaving our blue protagonists the only ones fighting the marines - how did that happen? Where did the water Na'vi disappear off to? (this is clearly an editing issue - btw, I hear Avatar 3 is 9 hours long as of this moment - all sfx will be added in and then the film will be cut to 3 hours - so similar editing issues are likely, if not more so)
There are so many dodgy writing and editing bits that by the end of it all, if you ever spend any amount of time thinking about it, you'll come to realize that your brain has been assaulted by incompetence and idiocracy time and time again.
Who is this film for, I wonder? Shouldn't it target people who are older/more mature than the first film (considering that 12 years have passed since the original)? It's surreal to think that the character depiction and their motivations have been stripped of any semblance of complexity which was present in the first film (even if it was present in the first film only in small amounts). Point being - this film is substantially dumber than the first film in terms of story. I would not recommend this film to.... well, pretty much anyone that cares about a decent story and/or well developed and deep characters.
Full disclosure: I saw this film in 2D - so I was paying a lot more attention to the story/characters than the 3D/sfx effects. I will say this about the visuals - na'vi look like videogame characters - not photorealistic. I haven't seen any improvements from the first film in that regard - only the difference being, this film focused on the na'vi world like 95 percent of the time, so the crappy graphics stood out a lot more than they did in the first film. Digital sfx do look better in 3D though, so I suppose it's possible that focusing on na'vi in this film may have improved the 3D experience.