I have no idea how this looked to someone who has never seen the series, but for me it was a big pile of shit and stickle bricks.
Don't get me wrong, what it did right was rare and beautiful, a glimpse of a much, much better film. But it just did so many things wrong. Weirdly wrong. Amateurish, even. Something that bears a far stronger resemblance to what would happen if you gave the people who made that weird Firefly fan film enough funding to buy Mars than if Christopher Nolan started making seriously bad directing decisions. About half of the movie plays like someone put together a clip show on youtube titled "Avatar Season One In 10 Seconds"- oddly jerky, with a vague feeling that what you're watching would make more sense in context, always cutting away before anything climactic or interesting happens in a somewhat arbitrary way. When the movie does recite nearly scene for scene for the show, it gets a whole lot better, and it seems to realize this. But there's simply not enough time in one movie to do that, and so a whole lot gets taken away. Events lose their significance; the revelation that Zuko was the Blue Demon is neither shocking nor particularly exciting as we had no base for how off-character this is for Zuko.
The costumes were really nice, though. I think maybe if the speakers had given out it would have made for a good movie.
PS: In this weak firebenders can only bend existing fires (torches, lanterns, wood fires etc.), which I found odd, stupid, and completely unnecessary. I suspect that this had something to do with trying to make the solar eclipse's importance more evident to those who've never seen the show, but in a movie this confusing it seems a bit arbitrary to dumb that part down when everything that should have been dumbed down was kept uncut or left unmentioned.
PPS: And before I forget, the casting was pretty weird too. Here you had whole villages of people who lived in pretty much an Inuit society, where all the extras look pretty native, but anyone important to (what I would hesitantly call) the plot is white as the driven snow. Aasif Manvi was good, though.
So, in conclusion: a lot of this movie just didn't make sense. The story, the characters, the cut-aways, the casting... but most of all, why it came out this way. Did NO ONE pre-read Shyamalan's script (including Shyamalan)? Good god, it was like Avatar as penned by Timmy, age 8. If you want to see it, don't. Just go watch the animated series again, I'm pretty sure it's an instant watch on Netflix.