They took an action comedy and turned it into a drama so there is very little to no comedy.
They took the characters, locations, and story line and tossed everything into a blender and now its rushed and confusing with characters popping up in weird spots, and events happening in different locations.
The supporting characters have no background other than to help the story progress so everything is 2d with them.
The main characters automatically know things they weren't supposed to leaving plot holes and unnatural interactions.
The Avatar isn't learning about the world through actual trials but just reacting to things that randomly happen.