In addition to what others said, it’s an extremely well fleshed out fantasy world deeply engrained in ancient Asian cultures, which is something most western media hardly touches. All four elements are based on different parts of Asia: japan, different Chinese dynasties, the Inuits, and nomadic temples. There’s sprawling cities with seedy lowlifes and political conspiracies, small fishing villages next to bamboo forests and icy mountains, and misty mountaintops straight out of Chinese paintings where monks meditate. Aesthetically, both visually and musically, it’s extremely pleasant to watch.
Not only are the four nations based on different Asian cultures, Each style of bending uses a different real world martial art in combat. Fire bending is aggressive Kung fu, whereas water bending is a graceful tai chi motion. Bending and fight choreography is extremely well done and based off of actual martial artists. The actual bending is endlessly creative, with every element having more powerful offshoots harnessed by grandmasters and prodigies (firebending, for example, can become lightning by a select few highly trained royals). Waterbenders fight with water arms, freezing opponents, flinging shards of ice, even manipulating the water in people’s blood etc. earthbenders have several variations including sand and metal. Aang the avatar can go into the avatar state when angry, a heightened rage mode with all his bending set to max strength, where his blasts of air can literally turn rock into sand within seconds.
Also, the characters are really excellent. Zuko in particular is a fan favorite for an extremely compelling and satisfying character arc from tortured villain to an extremely dimensional antihero. His relationship with his uncle Iroh is actually heartwarming, and its conclusion literally makes me sob like a baby every time I watch it. Character dynamics are believable and fleshed out, each character gets a unique relationship with all the others, and there isn’t a single character that feels like a drag.
I seriously can’t recommend this show enough. I mean, there’s a reason a show that’s ten or fifteen years old is currently #1 on Netflix right now. This is far beyond other Nickelodeon and Cartoon Network fare such as powerpuff girls, Ben ten, etc. Season one is more little kiddy, but once you get to season 2 it becomes utterly fantastic. Please watch this show now, you’re honestly doing yourself a disservice by not experiencing it for the first time.