For me it’s how clearly Zuko’s family story was told. Note it’s been years since I watched the original, but I remember it being a little muddled until later in the show. But it might have been the longer story spacing out the emotional impacts. The clear picture of a narcissist dad playing his kids against each other/their Mother, oof that hit close to home. Too real and relatable.

For me, I really like the changes they made to Kyoshi Island and how Suki and Sokka interact from the get-go. Even though they still could have done the scene of Sokka dressing as a Kyoshi warrior.

Zuko's crew being the original crew of the doomed mission he objected to.

It’s difficult to call it a change but I enjoyed the new version of the secret tunnel song. The accompaniment removed some of the poor traveling musician feel of the original, but it was enjoyable on its own merits.

Firebending being realistic (ie, actually burning people). That was the one that stuck out most, but in fact, except earthbending, it was cool to see all the new bending in live action.