How do you judge an entire TV series? It's easier with movies: either the movie sucks or it didn't. Maybe part of it is good and part isn't, but the sucky parts still affect the quality of the whole.
A TV series is episodic, even if there is a Myth Arc in play. One episode can blow, while another can be great. Over the course of 22+ hours of television, how much do you weigh the crappy episodes compared to the good ones?
The weighting system I use to justify my preference for DS 9 over all other Star Trek series is the Random Episode Principle: if you sat me down and forced me to watch an episode selected at random from a series, which series do you pick? You can't necessarily pick the series with the best episodes, because if the ratio of good to bad is 1:10, you're still very likely to get a bad one. This method forces you to pick the series with the most consistent quality, not necessarily the one with the best episodes.
Avatar: The Last Airbender is a quality show. It certainly has a curve to it, and if you're Anime-averse, it will be difficult to get through the early season 1 crap. But once the writers start settling in, it gets better.
It certainly has its problems. The overall plot is formulaic and cliché. The writing in many episodes descends too much into comedic elements to take the serious parts seriously. The editing often damages in-episode narrative momentum. The series often puts the breaks on narrative momentum with breather episodes in places where they don't help the flow of the series. Major turning points in the series are oddly paced; season 2's ending was faster than necessary, while season 3's ending was slow enough that it needed padding to fill out its 4 episode arc. And the writers often have promising lead-ins but weaker conclusions.
Yet the pluses do ultimately outweigh the minuses. The world is well-devised, with the integration of bending seen in very reasonable and logical ways of each of the bending nations.The action is usually strong and compelling, and the animation and music are well above average. While the plot is formulaic in overall structure, it is executed reasonably well within that structure.
But the main attraction is the characters. They are usually effectively written, creating in most cases actual, consistent people rather than comical caricatures. Character is even shown during action scenes; most characters have a specific fighting style that is easy to pick out from a crowd.
Most characters have a well-defined character arc through the story, growing and changing as time progresses. Aang's arc is generally about responsibility, though the ending can be seen as a betrayal of that premise. Zuko's arc is about learning who's support he should value, as well as about knowing himself and what the right thing to do is. Sokka's arc is focused on leadership and learning to trust himself. Katara's arc is about empowerment, and even shows the dangers that empowerment with the wrong mindset can bring.
Even side characters are shown to be interesting. The writers spent an entire episode chronicling the exploits of our villains... on vacation. Effort is generally spent on giving time and meaningful interactions to background characters.* This extends to the artwork as well, as background characters tend to have distinctive looks, rather than just being faceless masses.
While I would say that the show doesn't live up to the hype that some of its more rabid fans have given to it, it certainly does stand as a quality show.
The Future
So, there's a sequel series coming, as well as a comic book series.
On the latter, I'm generally terrified of comic book continuations of shows, though to be fair, that may be due to exposure to Buffy: Season 8. It seems like comic continuations want to do all of those things that couldn't have been done in the actual show. In so doing, they often betray the very things that the audience liked about the original.
For example, I could see them going really cynical in the comic series. No longer fettered by the needs of a kid's show, I could easily imagine that they'll incorporate all manor of things. I'm guessing that the Chess logic of the Fire Nation will quickly go out the window, so Zuko will have to deal with a civil war, especially if Azula gets her head on straight and breaks out. They'll probably detail the issues of the Earth Kingdom trying to deal with the Fire Nation colonies, many of which may have integrated so far that they simply don't want to be ruled by the Earth King anymore.
My main concern with the comic is that it won't really be able to maintain the themes and tone of the series. That would be fine if they went with a different overall story, perhaps by jumping ahead several years or so. But chronicling the immediate fallout of the series doesn't seem like it would have a voice of its own. The other major concern is with characters; after all, the show is more about character than plot. The Gaang have completed their character arcs (those that had character, of course); while it might be interesting plot-wise to have a continuation, I don't think it would be interesting character-wise. So unless it's about Toph Bei Fong finally getting some screen-time, or set in a different time period so that the characters can change off-screen, I'm ambivalent about it.
I'm more hopeful for the sequel series: The Legend of Korra (I patently refuse to stick that "The Last Avatar" on the front of it). I know very little about it; only the initial dump of info. This is by choice (so don't post any comments detailing spoilers, on pain of death), but what I do know makes it seem like a very different series from ATLA.
Avatar: The Last Airbender is more or less a road show. The Gaang goes from place to place; generally, once they've left somewhere, they're not coming back. Korra seems to be about a specific place. And that is good.
Why? Because the thing the writers are best at is character, not plot. And if you're constantly meeting new characters, you don't have much time to develop them. Aang having a Gaang was necessary, so that there would be at least someone that he hung out with consistently from episode to episode. Korra doesn't need a traveling Gaang of her own, because she's not really going anywhere. She can keep to the same area, where we can have a hefty suite of recurring characters rather than a small set of series regulars. The character development can be spread out among more people, rather than being focused on a small few.
I can't say I like the idea of her having learned 2 of her non-natural elements by age 16. Especially glossing over her training in her elemental opposite. Mostly because it makes Roku look that much worse, having taken 12 years to learn all three. Aang should have been the outlier, not the standard.
I do like the idea of non-benders vs. benders, in part because it suggests that Aang's actions had consequences he did not fully expect. In the finale, Aang showed two things: that he would not kill no matter what, and that he could de-power benders. While that would certainly scare a population of firebenders into submission, if I'm a non-bender, what is he going to do to me? Oh sure, he can fight me with his super-special awesome Avatar powers and bending. But he's not going to kill me. He may lock me away, but some of my friends can always bust me out. And we'll start the whole thing all over again.
If the worst thing that the fabled Avatar of legend will do to those who threaten the world is to depower them, then all those who threaten the world need to do is not have powers. And if you get a lot of them together, then what you have is a revolution that the Avatar can't stop. He can't use the threat of death to stop you, since you both know that Thou Shall Not Kill is in effect. And there will always be someone to break you out of prison. Therefore, an army of non-benders would have little to fear from Avatar Aang. He could stop them from conquering an area, perhaps. But little more than that.
I'm curious to see how this all works out. It could suck, or it could be awesome.