I like the way that characters respond immediately to sudden situations
I just finished a rewatch of the series after not watching it for a long time. I noticed that, in startling situations, where most shows would dedicate a shot to focusing on the characters' facial/body language to communicate their thoughts or emotions, this show often instead depicts its characters reacting instantly.
A few examples of what I'm talking about:
In "The Spirit World", when Sokka is taken by the Hei Bai spirit, Aang takes only a split second before throwing his glider and beginning to follow.
In "The Blue Spirit", when Zhao tells his men that the Avatar must be captured alive, Zuko instantly crosses his swords over Aang's throat.
In "The Serpant's Pass", when the group is travelling underwater in the bubble created by Katara and Aang, and the bubble is pierced by the serpant, Toph instantly raises the ground beneath them to the surface.
I'm sure there are more.
I actually think this creates a really cool effect, where the viewer is startled and impressed by the character's quick reactions. I specifically remember thinking the Blue Spirit example was super cool when I was a kid. He heard Zhao's words and immediately knew what to do.
I'm curious to hear if other people have noticed or appreciated this, or if anyone can point out other examples I haven't mentioned.