Shyamalan's The Last Airbender attempts to retell the events of the first season of the popular Nickelodeon cartoon Avatar: The Last Airbender, which by itself should tell you most of what you need to know about why this movie is so hated. The movie only covers about half the episodes, which are only about 20 minutes long after you remove commercials, the intro sequence, and the credits, but that still leaves almost three and a half hours of material which this movie tries to cover in half that time.
Maintaining the story through that process meant sacrificing a lot of understandability. If you're the kind of person who reads a blog like this you've probably seen the series and if not then you should, so I won't go into the story right now. What I will say is the editing process cut so much out that even the title character, Aang, ends up as just a plot device instead of a character, though to be fair even the characters that do stay characters have more wood in them than a 2x4.
By far the biggest mistake Shyamalan made, however, was the same one made by most directors shooting adaptations: he felt that he needed to "fix" the source material. The series was great but it was still a Nickelodeon production, that is to say aimed primarily at kids. It's more comedy than drama and generally lighthearted. Clearly, Shyamalan felt that the world of Avatar would be better as a straight fantasy epic. In the process he ripped out the heart and soul of the story. I like high fantasy, I really do, but Last Airbender is not generic fantasy and shouldn't be treated as such.
What we're left with is a semi-coherent mess of a film, choked by random changes for change's own sake, even down to randomly changing the way characters' names are pronounced. (For example the comic relief Sokka, whose name is pronounced exactly how it's spelled in the show but is pronounced "Soak-a" in the movie.) Of course, much has also already been said about the random ethnicity reassignment - in the show everyone was chinese, while in the movie everyone is white except the evil Fire Nation who are now inexplicably Indian - Mr. Shyamalan's own people - but honestly the less said about that the better.
KR Rating:  HORRIBLE
When I was researching this article IMDb assured me that people who liked Shyamalan's Last Airbender also liked Eragon, thereby instantly proving the point of every person who has ever said anything bad about either movie.
On a more serious note, one enormous oversight pointed out by fans of the show is the exclusion of the Kyoshi Warriors. If you've seen the show you know who they are. If not I won't spoil it except to say they're a group of warriors who served a previous avatar and that after season one they become a very important part of the show. According to Mr. Shyamalan, their leader, a female warrior named Suki, was his favorite character. He even shot an entire hour-long segment revolving around them, but in the end he cut the entire thing because he knew he would never be able to do them justice.
This, of course, begs the question of why this stroke of conscience did not apply to the rest of the process. Of course you can't do justice to six and a half hours worth of show in an hour and 43 minutes. Even a Lord of the Rings style three hour epic might've had trouble, especially when the cast is filled with - and I'm sorry but it must be said - talentless hacks, with the few talented actors on hand all wildly miscast, most notable in the case of comedian and Daily Show correspondent Aasif Mandvi, who is for some reason cast as the main villain!
Shyamalan's Last Airbender was doomed from the start, but somehow ended up even worse than anyone could've suspected.