The visual effects of this movie are incredible, but unfortunately, that's the only positive thing I can say about it. Whenever the camera would pan a scene, the image would become very choppy.
The film forced the plot to progress--dialog did not allow the film to flow, especially when we were forced to discover that suddenly the brother and the princess fell in love in what was apparently a matter of weeks. These confusions were driven by a poorly written script. For example, the king asks the commander if he really thinks the banished prince was the attacker, and the commander responds, (simply) "Yes..." [end of scene]. AND THAT'S IT?!?! I literally started laughing.
The script is the worst element of the film, followed by the acting. I have to give the Avatar credit for this being his first film. However, if the production team spent so much money on the visual effects, why didn't they invest in a black belt who could also act (let alone the brother and princess...) The actress who played the sister wasn't that bad though.
In conclusion, Shyamalan is finished. Next time he wants to produce a film, he needs to hire a writer a different casting director. In fact, he would be better off to just stop making movies.