Avatar the Last-Airbender is one of the animation series I watched as a child that left a lasting impression on me. So much so that every once in a while I re-watch it just like I did now.
I love how this American made series has so much of both American and Japanese influence which is visible in both the characterization and the surrounding. The first book is mostly a series of character introductions where the characters don't evolve much while from the second book forward the story development really kicks in to gear.
I always love it when there are distinct 'houses' where all the characters belong to. In the avatar world that being the elements Earth, Water, Air and Fire . Just look at Katara and Toph both being Water-Tribe and Earth-Kingdom respectively. Just that alone tells so much about the characters and makes it easy to go deeper in to the characters personality because you already know a bit how most people of their respective element are. This makes it even more awesome when they break away from those distinct personality traits later on or your view of the personality traits belonging to an elemental nation changes. The best example of this is the Fire element, the Fire-Nation is depicted as cold hard killers with a fiery personality fueled by rage and anger with a short fuse while later on in the series the fire element gets explored as the element of inner strength, calmness and determination to accomplish tasks and desires. This makes for interesting character development within the series, sometimes by changing your perception of a character instead. Allot of this also has to do with people from the nation they belong to having to deal with how other people in the world see the nations and trying to break away from that.
Overall I really love this animated series and with well paced action, fun moments and sincere moral questions this show stays awesome even after watching it multiple times.