I am a fan of the film. Not a super raving fan, but simply one who welcomes it for its merits, points out its opportunities, and eagerly awaits the following installments. So if you're one of those fans who hate the film, then by all means do not read this review. This is my opinion and I will stand by it.
I will admit that I was a bit disappointed with the film. The pacing didn't jive with me, some of the acting was...off...and I was worried that non-fans of the show might get confused by the story.
But, I did not think the movie was bad because it deviated from the series a lot. In fact, the elements that I thought weighed the movie down was its adherence to the show.
First off, the movie would have been a lot better if they started the narrative during Aang's days 100 years ago. Because it followed the series too much, it started where the show started--when Aang's ice orb was uncovered. I would have started the movie with a brief narrative, similar to what Katara did, about the four nations, about the Avatar. I then would've showed Aang as a monk on the Southern Air temple, how he found out he was the Avatar, his relationship with Gyatso, how he ran away from home, and how he ended up in the ice. That way, the audience (the non-fans) would acclimate to him more. They could've made the moment Aang was frozen in ice and the Air nomads were being slaughtered a very dramatic moment. That way, when he returns to the temple and discovers what happened, we can feel his grief more. That way, rather than just hearing about it in Katara's voiceover.
Fans keep on complaining that Aang wasn't as goofy or happy as he was in the series. I don't agree. Why would someone who's out of his time, an orphan, and quite possibly the one responsible for the demise of his people be goofy and happy 20 minutes or so into the movie? Or even at the very end of the movie? He needs to deal with his grief and I think the movie portrayed that quite nicely. Aang went through the various stages of grief. He was initially in denial (he didn't want to believe Katara when she explained the destruction of his people, he didn't want to reveal himself as the Avatar), he was angry (he lashed out against the Fire Nation, starting rebellions and kicking ass), he was depressed (I know, I skipped the "bargaining" stage since I don't think that was in the movie), and then came acceptance. Like what Pakku said in the film, water is the element of change. Book One is an excellent medium to portray Aang's grief, until he finally accepts his role as the Avatar. That is why I had no qualms when he didn't fuse with the Ocean spirit and massacred the Fire Nation fleet. Like what the Dragon spirit (and Aang from the series) said, he is not a killer. He is a gentle soul. He has this awesome power and rather than lash out like an angry God, he merely demostrated his power to the enemy and let them retreat on their own.
I would have preferred, though, if Aang was shown crying whilst bending the ocean in the film's finalé. Crying not in a wailing kind of crying, but more like the way Aang cried in the series when he was opening his chakras and Guru Pathik had him face his grief and conquer it with love. That would have sealed it for me.
I also liked how they "powered down" the Firebenders. It has always been my opinion that the Firebenders are far too powerful with them being able to conjure and manipulate fire. I know fire is the element of power and all, but it doesn't seem plausible that there was balance among the four elements when firebending is this strong. By necessitating an external fire source, the movie was able to portray the real difference of each element. As we've already seen, water is a versatile element. It is bountiful, but its manipulation requires grace and control. Fire, on the other hand, is aggressive, hard, and very quick. And because of its weakness, it is expended rather quickly too. Breath control as a limiting factor for firebending from the series, for me, is insufficient because all the bending arts require their Benders to move around and thus loose their breaths.
I also have no complaints about the pronunciations of the names in the film. I am somewhat more biased towards Iroh as "I-row" though rather than "Ee-roh", but I don't hate the latter either. I also have no issue about the race of the nations...well, perhaps not as much as the others. I did have trouble liking the Northern tribe because it was too whitewashed for me. Or rather, the people there seemed too...lavish looking. The city itself looked like a European castle and its inhabitants rich Feudal folk. Why would I feel sorry about these bunch of rich people when the Fire nation finally attacks them? They could've toned down the opulence of the Northern tribe and made its population more diverse. The Earth Kingdom was diverse, as was the Air Nomads. And I'm sure many White people didn't notice this, but the Fire nation was quite diverse as well. I just wish they added "brown" people into the mix like Mayans, Filipinos (I am one btw), Indonesians, Cambodians, etc.
I would also have changed Zuko's introduction to the Southern Water Tribe near the film's start. It seemed...I dunno...overly melodramatic somehow. Rather than him declaring who he is, I would've preferred that someone actually asked him who he was and then he made his announcement. I did find his scar to be too small compared to his cartoon self, but then again, scars do heal overtime so it would be completely ridiculous for his face to be too scarred. Besides, that will hide Dev Patel's cute face too much. My other issue is with Sokka's acting. I wish he'd smile more and crack a joke or two every once in a while. And the scene on the Earth kingdom village (with the cutest and littlest version of Haru) seemed...corny. I'm not sure how I would've tweaked it (and believe me I've thought about it many times) but I still cringe a little when I remember the scene.
Overall, I am looking forward to the next film, especially with great prospects like Azula, Toph, Suki and the Kyoshi Warriors, and the awesome city of Ba Sing Se. Just thinking about seeing the "live action" version of that city gives me goosebumps!