2. Mai and Ty Lee will appear
In addition to Princess Azula showing up earlier, we'll also meet her friends and teammates in season 1: the gloomy Mai and the acrobatic Ty Lee, played by Thalia Tran and Momona Tamada respectively. In the original show, we don't meet them until season 2.
I'm not expecting to see a whole lot of Mai and Ty Lee in season 1 since there are so many more important characters to develop. That being said, I suppose it can't hurt to foreshadow Zuko and Mai's relationship and Ty Lee's chi-blocking abilities early on. Azula, Mai and Ty Lee are the main villains of season 2, so becoming familiar with their motives and abilities will help new audience members get to know them.
"If anything, Azula's story in the first season is a little bit of a prequel to her story in the second and third seasons, but that's another element that we thought we should see rather than just talk about," Albert Kim said in an interview.
3. We will see the Fire Nation attack on the Southern Air Temple
In the original animated show, Aang runs away from his responsibilities as the Avatar, gets frozen in an iceberg, and wakes up a hundred years later to find that the Fire Nation is waging an expansionist war on the rest of the world. While he was frozen, the Fire Nation attacked on the Air Temples and wiped out the airbenders, Aang's people. Aang visits the Southern Air Temple early in the series to find the body of his old mentor Monk Gyatso surrounded by dead Fire Nation soldiers. That's the closest we come to see the actual conflict.
However, we know from the trailers for the live-action show that we will see airbenders defend themselves from the Fire Nation. The trailer even shows us Firelord Sozin — the ancestor of Firelord Ozai — leading his surprise attack to kill the next Avatar, which he knew would be an airbender. It will be interesting to see if the show portrays firebenders whose powers have been enhanced thanks to the power of Sozin's Comet, which is passing by at the time. Apparently Sozin's Comet won't be as big of a factor on the live-action show, which we'll get to in a bit.
While I'm not excited to see the airbenders get wrecked, it will drive home the fact that the Fire Nation started this war in a brutal way and 100 years later continue to subjugate the world for their own gains.