As with the characters, comparing Avatar the Last Airbender with The Mysterious Cities of Goldleaves me with the impression that Last Airbender is far more complex and sophisticated in its storytelling.
This is not to knock The Mysterious Cities of Gold, which was a welcome change from the episodic, repetitive storytelling of the time. Most of MCoG's contemporaries would establish a situation (Inspector Gadget), perhaps even hint at a mythology or canon (He-Man), but they never went anywhere with it. The world of the average childrens cartoon was static. The Fate of the Universe may well be at stake, but don't expect a resolution anytime soon. There was a real Roadrunner feel to the way the heroes and villains would repeatedly hurl themselves at each other, then fall away at the end of a half-hour, each vowing that things would be different next time. Though I can argue that it wasn't a satisfactory payoff - little green men? really? - Mysterious Cities of Gold had a serialized story that moved forward instead of in circles.
I mentioned a couple of times in the Quick Watch Guide that it felt like MCoG was spinning things out, adding things in the middle for no good reason. Part of that is because the pieces were a bit interchangeable. Certain plot elements had to be covered: the truth about Esteban's parents, the fate of Zia's father, and what happened to Tao's people needed to be addressed at some point. The order that happened wasn't crucial. If Zia had found her father in her home village, for example, the story would go on in much the same fashion. We didn't need to know who Esteban's parents were before anything else could happen. It was a side quest.
Compare that with Avatar the Last Airbender, where Aang must visit the different nations and learn the elements in a certain order. Beneath the arbitrary rules of the Avatar cycle, there is a deliberate movement from the "Primitive" Water Tribe to the more advanced Earth Kingdom to the industrial Fire Nation.
Or consider Zuko's story: he must pursue Aang while details about his past are revealed to increase audience sympathy for him, then be turned out by his family and learn to question his destiny. Eventually, Zuko had to return to his family and see for himself his father's evil before committing completely to Aang's quest. Both for the characters and story, these events have to transpire in order or they won't work.
Furthermore, early Last Airbender episodes like "King of Omashu" and "Warriors of Kyoshi" seemed to be about world-building and getting everyone set up, only to become very important later on as Bumi and Suki became recurring characters. Of the first season episodes, only "The Great Divide" is completely superfluous, without any callback, recurrent character or element. (I had thought "The Waterbending Scroll" was useless, but it establishes Iroh's determination that the White Lotus tile is crucial to his set.) Once the MCoG group acquires the Golden Condor a little before episode twenty, they are off to a completely different part of the world, leaving behind most everyone they've met so far for good.
It is a genre convention for the first set of villains to wind up downgraded to annoyance, for the second set to be rather vicious, and for the third set to be the true antagonists. On MCoG, Gaspar and Gomez get left behind in favor of the Doctor and Marinche, who don't really accomplish much, until the Olmecs take us through to the climax.
Last Airbender redeems Zuko, allows Azula her victory, then pulls out the Big Kahuna, Fire Lord Ozai. Where Last Airbender succeeds is in making their villains connected. Not only because they are family, but also because all three are in play in the final episodes. MCoG's Gomez and Gaspar and Doctor and Marinche are largely swept aside in favor of the Olmecs. In fact, aside from the three children, Mendoza is really the only character that carries through the whole series. (Technically, Sancho and Pedro count as well, but they are such sidekick cliches that I can't take them seriously as "characters.")
I think that the three nations of Last Airbender gave it an obvious overarching structure, one per season. Then, the rules that have grown up regarding the building of a season arc came into play. The half-mark two parter ("The Spirit World" / "Avatar Roku", "The Library" / "The Desert", "Day of Black Sun") would be a high point that would push the story in a new direction, then there would be a steady build to the season finale.
It's fascinating (to me) that you can roughly divide The Mysterious Cities of Gold into "seasons," even if the pacing is not as precise as Last Airbender. The finding of the Golden Condor at about episode eighteen/twenty is a natural break point for "Season One." Even better, that would put the group's arrival at the City of the High Peak and reveal about the medallion as the half-mark equivalent of "Avatar Roku" and the reveal about the comet. The second season, then, would start with "The Amazons," hit a high point at the escape from the Olmecs, then barral on through to the discovery and opening of the City of Gold.
Even though MCoG was conceived twenty years ago outside the standard American season structure, the conventions still hold. Cool huh?