Well let's see how far this goes:
Ubern00b reviews Avatar: The Last AirbenderIt's tough to watch a show that's already completed its run and judge each episode on its own merits, so I make no guarantees I won't be colored by my knowledge of future plot elements. Still, I'll try my best. Also: no plot recap. We all know what happens or we wouldn't be here, so I'll just focus on my opinions of events.
What really struck a chord with me regarding the show and it still carries with it today is the ambition behind it and that's made evident immediately in the shows opening sequence. The iconic camera dolly over the mountains towards the massive Fire Nation army would be impressive in a theatrically released animated film. Being an animator myself, it was nothing short of a miracle to see this kind of storytelling applied to animation after having to suffer through miles of crappy gag cartoons like My Gym Partner's a Monkey*.
And it just keeps getting better as I'm introduced to Katara and Sokka via the wonderful animation of Ryu Ki Hyun who does damn near half the animation of the pilot. His distinct style is something sorely missed in later seasons as he tends to draw slightly off model, but compensates with the expressiveness in the animation creating incredible amounts of subtle characterizations. One of the earliest examples is shown here:

During this sequence, Aang bats away at the spear twice with his hand, once to push it out of the way and the second time, he's absent mindedly swiping at open air, not once ever looking in the direction of the spear. What difference does that make? The gesture - though small - is a naturally occurring and understood behavior in
real people indicating indifference. The fact that Aang behaves this way towards a complete stranger wielding a lethal weapon speak volumes about his character in less than a second of animation. Moments like these pepper the pilot and the first season in general.
With regards to writing...there's honestly not much to go on at this point. Sokka, Katara and Aang start out pretty stock, with little time to establish themselves beyond their introductions, though special mention must be made to Sokka's moments of dry wit. These moments are so well-executed by DeSena's delivery that it actually seems like a break from character when he acts like a standard inept comic-relief meathead during the Zuko battle. Speaking of which...
I was really impressed with how the show introduced Zuko. While he carries as much impatience as any other standard villain, he shows himself to be a very
thoughtful one, not given to impulsive behavior. He demands to learn more advanced fire-bending from his uncle not for the power it offers, but because he understands the avatar (as far as he knows) would be a master of all the elements and he'll need all the help he can get to hold his own. He also openly acknowledges his defeat at the end of the pilot, inferring he understands at least the basics of learning from his mistakes...namely, admitting that he made some.
Overall, it's an impressive pilot punctuated by moments of incredibly nuanced animation and held together by great dialogue.
*To the fans of MGPaM, I just want to let you know: You're fans of a really sh*tty show.
Ep 103 review.