Take the first word from the first quote of the first episode "The Boy in the Iceberg". Then, take the last word in the last quote in the last episode "Sozin's Comet, Part 4: アバター Aang". There is a message :)
First Quote of the First Episode:
It's not getting away from me this time.
Last Quote of the Last Episode:
I think あなた all look perfect.
Form a quote from the underlined words.
The series gave us a message.