What sounds like the problem is with AIRBENDER (other than the well-established fact that MNS has no idea how human beings speak or behave) is that, like WATCHMEN -- which I liked but thought could have been better as a movie -- the focus was on making sure that all the iconic scenes got into the movie, because the fans wanted to see them. So we end up racing from one plot point to the next without getting a good sense of what it means or why we should care.
WATCHMEN kind of worked even with that in mind, but it was also twice as long as AIRBENDER's running time. There just isn't enough time to do everything in a movie -- that would be true if it were a three hour movie (which it should have been) but it was literally the worst possible way to do the movie in 90-some minutes.
I don't know what remained and what was cut in the film, but I know you could cut Aunt Wu, cut Omashu and Bumi, and probably (though not without some difficulty later) cut Jet. You could also cut Haru and the Earth Kingdom prisoners, but they might be useful in terms of condensing the story by having Admiral Zhao be there, instead of Admiral Sulu.
What you cannot cut are the Kyoshi warriors, the Southern Air Temple and Northern Air Temple (although you could probably combine them and have all the necessary events take place at one or the other), meeting Avatar Roku, and the events at the North Pole.
Three hours would be tough, but it would be doable. The bigger issue is having time for each character to be a character rather than a pawn moved around making plot happen. That's what you really need the time for.
The problem with Shyamalan is that early on, he was insecure and worked hard to show he knew what he was doing, which meant that he put together something great because he was constantly second-guessing himself and making sure he was making the best choices.
But then after SIXTH SENSE, he had all of Hollywood telling him he was a total genius who could do no wrong, and he eventually started to believe that this was true, and stopped second-guessing his choices because he had been convinced they were self-evidently genius.
He used to work hard, but he made it look easy. Then he got convinced that it WAS easy and stopped trying, and stopped listening to any dissenting voices. If they don't like it, that must be their problem, because he's a genius you see.
I don't need to say the other name we're all thinking.