The original script had Azula vaporize the ship captain after he mistakenly refers to Zuko and Iroh as prisoners, thus blowing Azula's cover story. This was changed because the creators deemed it to be too intense for younger viewers.
While Aang has already gone into the Avatar State several times, this is the first time it's actually referred to as such.
With this episode, Aang has gone into the Avatar State in every one of the four nations: the first time he changed was in the Air Temple when he discovered Monk Gyatso's body, then in the Fire Nation Avatar temple, then in the Northern Water Tribe, and now finally in the Earth Kingdom.
This is Azula's first speaking appearance.
Avatar Extras says that the Fire Nation spa used to be a resort for Earth Kingdom monks before it was conquered.