The first movie had about as average of a plot as you can get.
It wasn't terrible by any means; but it certainly wasn't great by any means, either.
If you remember back to before Avatar 1, one of the movie's main drawing-points was the technology involved. It was supposed to revolutionize CGI, and bring in a new era of 3D movies. It was supposed to be as big of an upgrade as you can get, in terms of the movie theater experience.
Another huge selling-point was that it was from the director of Titanic and Terminator. The actual storyline of the movie was really not that involved, in terms of the marketing (and, I think, for good reason).
Now that we know the storyline is pretty boring, and the visuals aren't actually that crazy, and Cameron's cinematography skills aren't quite what they used to be... I'm not sure how the sequels will turn out.
I suspect the advertising and hype will cause Avatwo to do quite well. But I also suspect that, with the sequel, it will become apparent that this series isn't all that compelling. So, for the sequels beyond, we'll see....
The concept art they've released so far looks downright laughable.
The 'screencap'/'art' in the same page of this same thread looks even worse than laughable.
I do actually hope the series does well. I feel like Avatar has a lot of potential, if they actually go crazy with it and develop a whole, intricate plot-line.
I'm just, realistically-speaking, not sure if that's a feasible objective for this series. I don't know the the powers-that-be have the skills/time/effort/interest to actually make this something special.