As someone who has seen this show multiple times, I can easily say that this one of my favorite cartoons ever. The characters on this show are very interesting and well developed, each one of them having a certain history shaping them into the characters that they are such as Katara whose mother was killed in a fire nation raid which lead to her (and Sokka) developing antagonistic feelings towards anyone from the fire nation or Toph (my favorite character) whose blindness caused her to be constantly pampered by her parents which eventually caused her to develop a rebellious attitude because her parents never let her have any real freedom. All of the main characters are very interesting in their own ways and it will eventually lead you into liking all of them.
The animation is very well done for the time this show was released. The characters are detailed very nicely and the bending choreography as well the martial arts mixed with them is well executed. The music for this show is great to listen to and I even have a few tracks on my iPod lol.
Like another user said, this show has a perfect balance of action and comedy. Although there are a few filler episodes, they are very fun to watch and add a little bit of character develop in the process. If I had to grade the three seasons, It would be like this:
Book 1 - 9/10: A great start to the series
Book 2 - 9.5/10: Better than the first book and it introduced my favorite character
Book 3 - 10/10: The final battle against the fire lord is one of the best fights I have seen in a cartoon
All in all, ATLA is definitely one of best cartoons I have ever seen and I'm really glad that Mike and Bryan decided to make a sequel series to this because more Avatar is always awesome to me.