All I was going to say in response to assertions that the movie was predictable and the ending could have been stronger was that I can't really think how they could have ended it any other way.
Because the whole human presence on Pandora was based on mining and company shares means that, if that presence is no longer profitable, the company'll just pull the plug. All you're going to get are some meaningful looks from the soft corporate representative (which we did).
If it had been more of a human exploration/expansion situation, you'd have more wiggle room in terms of shared existence and learning. But then you wouldn't be able to rely on the plot device: "The-crap-we-want-is-under-their-house-so-we-have-to-blow-it-up". Which means you'd have to make the Na'Vi more unbending and aggressive which would, in turn, make them less likable and you wouldn't be rooting for them.
I think they had a concept: "Future-Humans-Explore-Neon-Jungle-Planet-And-Meet-The-Natives-In-Avatars". It was probably even simpler than that, but mebbe I'll give them the benefit of the doubt.
The rest was contrived to give the concept a supporting plot and arch which would (hopefully) justify the massive amounts of money needed to bring such a concept to life and to do it justice.
Would it be nice if the story were more original and eye-opening? Perhaps. But those aren't the kinds of movies Cameron makes. He makes popcorn movies. Probably his most thoughtful movie is "The Abyss". He tried to get interesting about the underwater beings; but people were more interested in how Ed Harris was able to breathe underwater than cross-species diplomacy.
Avatar does exactly what it was intended to do. It entertains, taking CG and 3D technology in the film medium to a new level.
The fact that it trips down the well-trod paths of white guilt, battles over territory and indigenous people versus oppressive invader is less a fault and more of an understandable compromise.
The movie was nearly three hours long because they had to introduce the audience to the huge concept and do it justice. Because the concept demanded so much time, the plot had to be presented in big, easy to digest chunks. Personally, I don't think the movie felt long because, despite it's length, they had to charge through a ton of material even with the minimal effort given to developing a new and original story.
In order to get a more detailed and "interesting" plot, this would've had to be a trilogy (I say that instead of of two-movie arc because I don't think Hollywood makes just sequels anymore, not when there's a planned progression). They'd have to spend the entire first movie introducing the audience to Pandora and the concept of Avatars and how neat the 3D stuff is. The second movie'd be spent embedding Sully's character with the locals and having a downer ending for the middle act. And the third would be the return of the Na'Vi. But then, that's beginning to sound familiar as well, isn't it?
Look at District 9. That was another big concept movie that also used some very familiar plot basics but made it infinitely more interesting and complicated than Cameron was willing to go with Avatar. And that story feels like it's not done yet (a sequel is very probable, if only to prove my earlier assertion about sequels vs. trilogies wrong). Additionally, that movie started as an independent short film that managed to catch the eye of an individual who has a little bit of pull in the movie industry after making such light fare as the Lord of the Rings trilogy.
Again, I think this movie was enjoyable. I, personally, liked it a lot (enough to see it again in IMAX if the opportunity arises). The plot didn't blow us out of the water, but it really couldn't without making the entire affair too big and unwieldy to sell to the people who put down the money to make these things. Perhaps that's not an excuse or sufficient justification for some. But it's how I've explained it to myself.