I don't think I really have to explain...
If the show had never existed, and that was just some oddball idea that Shyamalan had, I would have honestly thougth it was a decent film. Not great by any stretch, but a decent action/fantasy. However, the show DOES exist, and was amazing. The movie not only gets some key things wrong, but it also lacks most of the heart and humor from the show. The charactes were just kind of shells of themselves, compared to what they're actually supposed to be.
I don't think M. Night is a bad director, in fact I like several of his earlier films. But I think one thing he has proven he is not, is a big budget blockbuster director. He excelled when he was working on character driven stories, in a smaller scale. Like Signs, that was pretty good I thought, and it dealt with a worldwide alien invasion, but only focused on one family's experience of that. And that was for the best. With After Earth, which wasn't a BAD movie, it was just kind of bland, I think the original story concept would have made for a much stronger film: a father and son who were camping/hiking in the wilderness, the father gets hurt, and the son has to go for help. That would have likely been a nice, character driven drama. Instead, Will Smith wanted to turn it into a sci-fi epic, which simply isn't M. Night's forte.
But yeah, The Last Airbender wasn't great. And honestly, while I was originally exicted about, they never should have bothered trying to adapt it to film in the first place. The show was perfect as is, they even shirked the usual Hollywood mold, and purposefully only made it three seasons, with a specific, pre-determined beginning, middle and ending. Unlike most modern shows, which fly by the seat of their pants, have little consistency, and often feel like they have no real concept where they're ultimately going a lot of the time.