I remember seeing the trailer and rolling my eyes. Mostly because it was just another American show trying to be anime. I hated it for Teen Titans, and I hated it for Totally Spies.
The episode that hooked me? Hmm...I remember being just mildly interested until the season finale of season 1. Then I refused to watch season 2, because I forgot it aired and when I tuned in, it was the episodes without Appa. Which I mostly hate. Mostly. Then...once again...the season finale of season 2 re-hooked me.
My first experience was I saw the last half of the Haru and the prison episode, and the scene that got me hooked was the exchange between the guards and the captain.
Captain: What was it? Guard 1: It looked like a giant flying bison Guard 2: A giant buffalo sir Captain: Well which was it? A buffalo, or a bison? Guard 1: Uhhh.....I'm not quite sure what the difference is sir.
Considering the Fire Nation has had its Prince, Princess, their Navy, their Army, their Airforce/Blimpforce, their Firelord and every almost every bounty hunter or mercenary group defeated by a child, albeit the Avatar, and a group of teenagers consisting of, Toph the blind Earthbending girl, a Waterbending girl who never had any official military training yet can best the toughest Firebenders and finally Sokka whose had at most a small amount of combat training, I wouldn't consider their spearmen to be top notch either.
I saw the first episode, I thought it was sorta meh. I kept watching and with each episode I became more and more hooked on the show. Around the time the Blue Spirit showed up, I was addicted to watching the show and knew i'd be a lifelong fan.
Well considering their initial target audience it makes sense. They just needed time to decide the theme for the series. My favorite thing about Aang's maturity is that his voice gets slightly deeper as the series goes on. First season Aang was the high-pitched pre-pubescent child that they were aiming for in the audience, but as the series gained a reputation and matured, so did Aang.
I watched the show from the premier, was pretty invested, and got to the The Day of the Black Sun, though I'm pretty sure I missed a few episodes in Season 2. For some unintelligible reason I stopped after that. Now, years later, I watched Legend of Korra, and got hooked, so I went back and re-watched every episode (just the other week, actually). It was glorious burning through the season, plus I got the bonus of moving straight to season 3 after the cliffhanger at the end of season.