What were you expecting from a sci-fi movie? Depth of plot? Profoundness? Artsy storytelling, perhaps? This isn't exactly The Notebook or A Very Long Engagement. It's a rollercoaster ride with stunning visual effects. You might want to lower your expectations a couple of light-years; then you wouldn't be so disappointed.
That kind of argument doesn't exactly work when the movie in question was billed as a "revolution." And, really, why should I lower my standards? There's plenty of really good sci-fi storytelling. In fact, T2 was a much, much better constructed story than
Avatar -- whose story was hardly worthy of credit, let alone analysis. I actually enjoyed
Avatar quite a bit (I actually enjoyed
The Phantom Menace quite a bit, too). But that doesn't mean that the films are above reproach or criticism for poorly constructed, paced, or executed storylines.
Kudos to Cameron for his world-building and the visual presentation of the film ... but he gets demerits for not paying as much attention to detail in the story as he did in the look of Pandora.