One of the problems with the movie is the lack of background development for the characters. We never learn anything significant about their past or get a feel about who they were back on Earth.
That's not a problem at all. Building character through backstory is one of the lamer, time-wasting ways to do it in a movie. The behavior of most of these people is enough to define them very well as people, which is as it should be - we spend our lives dealing mainly with people about whom we know little other than how they present themselves and interact with us and others in their current roles (and if you think otherwise you're simply unobservant).
Someone may have been dropped on their head, but it wouldn't be Jake Sully.
But building a character through backstory is certainly required if you don't build them during the present, which wasn't done in Avatar.
What was the colonel's motivation, a life-sucking space-alien wrapped around his spine? Did he just live to kill innocent women and children or something? Movies give more depth to Nazis than he got.
What was Grace's motivation? She's supposedly a
botanist, so what the heck is she doing trying to interact with the natives? She supposedly invented the Avatar program, so why was her specialty studying
plants? It makes no sense. If she was opposed to what the corporation was doing, pushing Na'vi off their land, why was she working for the corporation? The first hint we get that she's upset is when she founds out they'd set her up with a lame excuse for an infiltrator - Sully.
What was the company's motivation? Profit? But if they wanted profit they'd have spent time examining whether it was simply coincidence that the home tree was atop the largest deposit of unobtanium. Even a half-wit would wonder if the trees are acting as biological concentrators, as often happens with Earth plants (many plants are used to clean toxins from the soil or concentrate rare elements).
As for Sully's motivation, did he have any? As Lloyd George said about Lord Derby, he's "like a cushion who always bore the impress of the last man who sat on him." When the colonel was the last man who sat on him, Sully did the colonel's bidding. When Grace was the last to sit on him, he did Grace's bidding. When Neytiri was the last to sit, he did the Na'vi's bidding. Instead of being CGI, Sully was a character made of Play-Do.