Tenzin, Aang and Katara's son, is the only master of Airbending at the current time. He and his children are also the only known Airbenders following Aang's death.
The benders in the opening sequence are the four most recent Avatars. The Earthbender is Avatar Kyoshi, the Firebender is Avatar Roku, the Airbender is an older Avatar Aang, and the Waterbender is Korra.
Jinora asks Katara "What happened to Zuko's mom?" This line refers to an ongoing subplot in the original Avatar series that was never answered onscreen and has been asked by many fans.
The first season of this show is called "Book One: Air". This naming scheme follows the show's predecessor, "Avatar: The Last Airbender" who also named seasons after elements. Air was also the only one of the four elements not used for a season name in Avatar.
Meelo: Mommy, look! I'm a snowbender! (Blasts snow into Pema's face)
Tenzin: Earth. Fire. Air. Water. When I was a boy, my father, Avatar Aang, told me the story of how he and his friends heroically ended the Hundred Year War. Avatar Aang and Fire Lord Zuko transformed the Fire Nation colonies into the United Republic of Nations, a society where benders and non-benders from all over the world, could live and thrive together in peace and harmony. They named the capital of this great land, Republic City. Avatar Aang accomplished many remarkable things in his life. But sadly, his time in this world came to an end. And like the cycle of the seasons, the cycle of the Avatar, began anew.
Korra: (Korra trying to persuade Tenzin why she should stay in Republic City) Republic City needs you, but it needs me too.
Pema: (Tenzin tries to help Pema down from the bison) Stop doting on me. I'm not helpless, I'm just pregnant.
Jinora: Gran Gran, I've been reading all about your old adventures. I've been dying to ask you....what happened to Zuko's mom!?
Jinora's question about Zuko's mother was purposely ignored. This is because several month after the first seasoned aired, Dark Horse comics and Nickelodeon created the second Avatar: The Last Airbender graphic Novel: The Search, which explains this.
This episode was available online on March 24, 2012, three weeks ahead of it's Nickelodeon premiere.
Republic City partially resembles old time New York City, including the statue of Aang on an island, similar to the Statue of Liberty.