"Shyamalan's true achievement in this film is that he takes a thrilling cult TV series,Avatar: The Last Airbender, and he systematically leaches all the personality and soul out of it — in order to create something generic enough to serve as a universal spoof of every epic, ever.".

quote from a critic, and I couldn't have put it better myself

#1 screwed up firebending- Iroh is the only firebender who can create fire

#2 screwed up the spirit world- AKA LEFT OUT ROKU, used a stupid dragon, and in the process screwed up the avatar "The avatar is not supposed to harm anyone" AIRBENDERS are taught not to harm, the avatar "does not have that luxury"

#3 No Suki, no Bumi, and again, NO ROKU, but LOTS of Ozai

#4 Comet arrives in 3 years (only ozai knows this, "ONG" and the others don't) and it allows fire benders to create fire...

#5 Aang is captured in the northern air temple, which is also where the avatar statues are....

#6 Aang goes into the avatar state at will at the end... no water spirit anger, barely any pakku...

#7 fire benders bow to aang with water benders after aang creates big harmless tidal wave to scare away fire nation

Now for good things-

Actors were good (given the terrible script they have to make believable), looked good.

Bending LOOKED good, except water bending took like an hour to start

Blue spirit was spot on

and they made iroh a beast

I'll be surprised if they sign on for another 2, it looks like it's gunna bomb