I think people who have complaints about Avatar’s script or story don’t realize what Cameron was trying to accomplish with his movie. It was never his intent to create a movie designed to give people a complex story to figure out or be surprised by. I’ve seen several interviews with Cameron, with the most interesting one being an interview with Charlie Rose on December 17, the day before the movie was released. In it, and in response to questions by Rose, he addresses most of what have turned out to be complaints about the movie here.
Cameron acknowledged first of all that he most definitely wanted people to take an environmental message from the movie. He also said that he felt people had enough information on the environment, having read the stats on AGW and seen An Inconvienent Truth and forth and that they didn’t need more in the way of information, they needed a way to relate to environmental concerns on an emotional level. So he set out to connect with them in such a way as to make environmental appreciation and concerns resonate with his viewers in that way, rather than by bombarding them with more information on the subject.
He also sought to convey a message about war. His view was that since this country has never been invaded, most of his audience has never experienced what it’s like to be invaded. So he wanted his audience to experience through the Na’vi the pain and horror and misery that result from having your homeland invaded like that. And he was doing that in an effort to try to get us to think twice before we decide it’s necessary to invade someone else. (And, contrary to what you might expect, he didn’t seem all that determined that we never invade another country, only that we should think it through and make damn sure that what we’re doing is the right thing to do before we inflict that pain and horror on someone else.)
And he was forthright in admitting to some aspects of the film that people in this thread have objected to, such as the striking similarity between the Na’vi and human beings, the weaponry used, and its intentional broad-based appeal. With regard to the Na’vi, he said that he needed the audience to be able to relate to the Na’vi and how they were living and what they were going through, and that to make them alien and bizarre would make it difficult for audiences to identify with them.
With regard to the weaponry and why it wasn’t more advanced, it’s pretty much the same thing: the audience’s ability to identify with it. He didn’t want his audiences being dazzled by whiz-bang weaponry that was unfamiliar and would take them away from experiencing the emotionality of the scenes in which weapons were being used. Everybody already knows how the weapons in the movie work, so they’re better able to focus on the damage being done rather than on the weapons themselves.
And with regard to making a movie so obviously intended to have mass appeal, he quite naturally wanted the largest number of people possible to be exposed to the messages the movie contained, but he also felt a moral obligation to the studio that had entrusted him with the huge amount of money necessary to make the film, and he wanted to make sure that investment got repaid. This frankness and financial concern was also evident in Cameron’s response when Charlie Rose asked why Neytiri had breasts. He said “Because we wanted to put male seats in the movie theater”. 
And then there were the extraordinary lengths Cameron went through to make the movie as believable as possible within the context of the movie as it was to be made. He hired a professor of linguistics from USC, I believe it was, to invent an authentic language for the Na’vi, even down to making sure the grammer was correct. He took the actors to Hawaii for three days to prowl around in the rain forest there in order to develop the sense memory of what it’s like to be in a jungle environment. The actors had to take archery lessons and horseriding lessons too, all in an effort to make sure that their performances rang true.
So, all in all, Cameron was juggling quite a few balls simultaneously. He wanted us to look at human beings from an outside perspective, to experience and indentify with a culture that revered its environment, and to experience the shock, horror and misery that people experience when they are invaded. Plus he needed to develop or invent new technology in order to make the film in the first place, and he needed to repay the investor who had trusted their money to him. And he accomplished each of them extraordinarily well in my opinion.
So in closing, there is nothing either in the movie or left out of it that wasn’t the direct result of a conscious decision by James Cameron as the film’s creator and director. He has created a film that is not only a technological marvel but whose story is resonating with people emotionally and breaking attendance records in virtually every corner of the world. In no way did he place all his focus on the technologicall aspects of the movie and then just throw in a slapdash script to go with it, which seems to be the common misperception among the film’s critics.