One of my all time favs. How to summarize, great plot, really great voice acting, good to great animation, memorable characters and great writing.
This was one of those series where you loved every part and couldn't wait for the new episodes each week. This show is why they invented popcorn. :)
***Spoilers from here on. If you just want the summary, skip to the last paragraph.***
Each character had life breathed into them and had depth beyond most any other series ever made (cartoon or live action). The acting was a bit childish, sure, but it was done so as to make the characters more human, which it did very well. Sokka was dorky, kinda stupid, a screwball, yet very cool, the kinda kid you'd want to hang with because he was so funny, even if it was an odd sense of humor. Aang was a fun, lovable, really bright kid, the most popular and talented kid we all wished to be like, but without the tremendous pressure to save the world.
The story was mostly unique, and kept about as perfect a pace as one could ask for. The only issue I had with the pacing was in the third season, at the end they seemed to conveniently wrap up a few things, just so they could keep the number of episodes in the season comparable to the first two, but it was by no means unforgivable.
The villains had huge depth of character, very well developed back stories and motivations, and each had their own agenda, which often did not coincide with other villain's agendas even though they were, in theory, on the same side. This provided for lots of conflict and realism as many times these agendas conflicted with each other. You don't find this kind of writing every day. Priceless.
You find throughout the series that the people who are supposed to be the villains are really not that bad at all. Some were uppity and self important sure, maybe a little brainwashed by government propaganda, but aren't we all like that to some extent? Almost every episode had extras with great history, and the telling of each one added greatly to fleshing out the cultures involved.
The issues the characters worked through during their journey presented some serious situations and topics, such as love, romance, loyalty, life and death, hate, self worth, and lots of morality issues. More than any other series I've seen, this one related to the human condition and the issues we face in life, all without being a heavy emo drama, which was very cool.
***End spoilers***
In the end, Avatar: The Last Airbender is a great series with great characters and an epic story that will last a very long time. Avatar is worth watching again and again. You won't regret it.