This episode took a while to grow onto me. It's actually a nice episode, exploring the background of firebending, introducing the original firebenders, having some much needed Aang and Zuko bonding, some backstory - it's all there. But there was something that kept bugging me in this episode, and I just couldn't put my finger on it. Once I did, it was okay.
There are two tasks this episode has to fulfill: Give us a good plot (because there are not a lot of episodes left to tie things up), and show Zuko's new role as part of the gang. The latter one is mostly addressed in the first part, showing the wide range of responses still present to Zuko's joining. And while Katara especially has not yet accepted him, he is starting to carve out a niche for himself, both as the group's firebender, and on a meta-level as the outsider trying to find his place. He's been around since the beginning, and now that he's joined, a whole new level of group dynamics just opened up.
While both of these are fulfilled adequately, the episode as a whole is not that good, especially when compared to what has come before. It's not that the dialog is off, or that the effects are bad, or that the story isn't there. Somehow, this episode lacks focus. It just sort of floats, bringing what it's supposed to, but not really presenting it with a lot of impact.
Not that what is actually shown is bad. We finally get to see Toph first learning earthbending, something that is adorable, hear about some original benders, and even get to follow Aang and Zuko into the ruins of a lost civilization, showing as a whole deal about the setting, Iroh's past, and we even get to see live dragons. During this, there is the growing trust and understanding between Aang and Zuko, even though Zuko talks a bit too much.
When we finally get to see ancient Sun Warriors, and get the whole encounter with the dragons, the theme of the episode turns into something majestic, like there is some much greater power out there, more powerful than even the Avatar himself, able to judge him and teach him. The music as well as the mere presence and excellent animation of the dragons support this.
However, for some reason, the episode is a bit too lighthearted for its own good. I'm guessing this particular mood was supposed to balance out the seriousness that permeated the last three episodes, but somehow the tone and the theme don't fit together too well. Sure, Zuko teaching Aang firebending could be viewed in a lighthearted way, but the exploration of the ruins, and the appearance of the dragons doesn't fit to that. Which is why, in essence, this episode feels sort of empty. I want to feel the grandness of the dragons, I want to feel the wonderful revelation that they impart of Aang and Zuko, but I can't. The mood was just not set for that.
So overall, while the episode covers some important topics and finally allows Aang to overcome his fear of firebending, this isn't a particularly strong episode. It's just nice. It has a deep message that lacks depth in the presentation, it has majestic scenes that lack the necessary emotional impact. And while it is a very nice episode to watch, it remains just that, and doesn't become something significant.