Visual effects are great for the most part. Fire and earth bending are amazing.
Only airbending leaves something to be desired.
Some odd choices narratively. It seems like they wasted their big pay off (the comet) in the very first episode.
Fairly poor acting (especially from Aang and Sokka).
However, the worst part of the show is just how much exposition there is. Everyone just keeps talking about whats happening instead of showing the audience. I highly suggest watching the non action scenes on 1.5x speed. It'll make it less unbearable.