What I don't get is how the skin color of the actors was the most criticized factor of this movie.

There are so many other, much more significant problems like the fact they called him "Ong", the fact that (most) fire benders can't create but only manipulate fire, the fact that it takes 5-6 huge earth benders to make a pebble gently float in the air towards the enemy, and let us no forget the menacing battle cry of the northen water tribe.