Actor Existence Limbo: After Mako's death, Iroh was relegated to only two silent appearances for the first half of season 3, before Greg Baldwin was introduced as his new voice.
Crossdressing Voice: While this was averted in the original version as Aang was voiced by a boy, it was played straight in Russia, the Philippines, Japan, Korea and Taiwan.
Dawson Casting: Aversion: Aang and Toph are voiced by actors of about the correct age. The actors who play Katara and Sokka are not much older than their characters, either.
Played straight in Chile with Aang, whose male voice actor (René Pinochet) was almost 30! Justified that he was born prematurely and his voice hadn't changed much since childhood. Dante Basco, the actor for Zuko, also had a young voice in his late 20s when he did voice work for the show.
Fan Community Nicknames: Due primarily to one certain "megafan"'s infamous, longrunning satire comic of the Avatar fandom, fans have come to almost unanimously accept the term "Avatards". (Some people feel that, in referring to the term "retard", fans' use of this label is demeaning to the mentally-disabled.)
The term fell out of use after The Legend Of Korra was announced, at least partially because of the Unfortunate Implications - it's now rarer to find someone in the fandom who calls themselves an Avatard than someone who doesn't.
Scott Menville (Robin, Ma-Ti, Lloyd Irving) is a representative of one of the two tribes looking to cross The Great Divide, the spooked Ba Sing Se University student in City of Walls and Secrets, as well as one the guys who hits on Ty Lee in The Beach. He does many random voices throughout the series.
What Could Have Been: Toph was originally conceived as a guy. When designing the show's lead Earthbender, who was originally supposed to be a teenage boy, head writer Aaron Ehasz joked that it would be funny to see a small little girl take out all those big, burly Earthbenders. Though the idea was rejected by series co-creator Bryan Konietzko, the team still decided to work around it, and several discussions and redesigns later, Toph was born (And ironically became one of Bryan's favorite characters). Variants of her original character model makes a few appearances throughout the series in bit roles, most notably as a bender in the show's opening sequence (Roku's Earthbending teacher) and THE BOULDER!
Word Of God says that the screechy actor who played her in the play is a flanderized version of her original character design. Aspects of her original male visual design and characterization were later incorporated into Bolin.
Speaking of THE BOULDER, the creators originally wanted him to be voiced by the Rock.
Incidentally, Azula was also originally conceived as a guy.
In the season 2 premiere, originally Azula was going to vaporize her captain for letting it slip that Zuko and Iroh were prisoners, but realized they couldn't do that on a kid's show.
See also Cliff Hanger on the main page. The production staff even planned a storyboard for it and everything. In the Avatar Extras for part one of the Season 1 finale, it is mentioned that there was an idea kicked around for a special about Iroh's failed attack on Ba Sing Se, but it never took off.
Trivia tidbits reveal that there were a number of storylines that were cut from the final plot such as: Momo being the reincarnation of Monk Gyatso, the identity of Aang's parents, and an episode that involved Aang and Sokka getting handcuffed together.
Originally, in Book Three, Azula was apparently meant to have been in an arranged marriage, and Appa, in the season finale, would have been confirmed as female. They decided not to do this, for obvious reasons.
Waterbending is the only bending skill that was not learned from an animal. Waterbenders are said to have learned from the moon; in contrast, earthbenders learned from the badger-moles, airbenders learned from the sky bison, and firebenders learned from the dragons. The entire concept is awfully similar to animal-based martial arts in Real Life, made famous by Hongkong martial arts movies like Jackie Chan's Snake in the Eagle's Shadow or Tiger and Crane Styles (best known for being edited into Kung Pow!.)
As mentioned on the main page, the bending styles themselves are based on actual Chinese martial arts (Waterbending is Tai Chi, Earthbending is Hung Gar, Firebending is Northern Shaolin Kung Fu, Airbending is Ba Gua). Exceptions not covered are:
Toph's own Earthbending style, taught directly by the badger-moles, is actually based on Southern Mantis style
Metalbending, based on Xing Yi Quan, which quite appropriately was developed in captivity as well
Bloodbending, based on Qin Na Shou, which was also quite appropriately a grapple-based technique
Redirecting lightning is the only style taken from a sword technique, the obscure Tai Chi sword discipline
Extra points for Lightning redirection being explicitly based on Waterbending techniques.
You can probably catch some differences between Katara and Pakku's waterbending styles, since Southern and Northern are legitimate Tai Chi variants.
"Katara" means "curse" (both as in "hex" and "swear") in Greek. Of course, this is just a weird coincidence, but it is too odd to not mention. One would suspect that the character was so named because a Katar is a type of Indian dagger, though.
Although the earthbender was Toph's original design, but when her design was changed, that design inspired later characters The Boulder, Sud (Roku's earthbending teacher) and "The Ember Island Players"' Toph (directly taken from that design).
Some argue that the airbender in the opening is Monk Gyatso. Others claim it's an older Aang (a theory largely debunked by the confirmed older Aang in the opening to The Legend Of Korra, who looks much different), and still others suggest it might be one of the unamed monks from "The Storm".