I'm not going to go into any great detail, but here are differences from the series. The whole thing seems rushed in parts, though I don't know whether that is just because I know the series so well. Things that took an episode to set up were over in minutes. There were a few changes that I likes and that I didn't. It was weird.
In the film and in the series, Aang runs away because he doesn't want to be the Avatar, but in the series it is because it is because he was supposed to be moved away from Monk Gyatso. In the film, the main problem for him seems to be that the Avatar isn't allowed to have a family. Um... What? In the series there is nothing that sugest this. In fact, Avatar Roku married and had a child, he was Zuko's great grandfather. So are they going to disregard that in the films? Or say that there are different rules in the Fire Nation? It seemed a bit pointless. I mean, I'm no expert, but I'd think that telling the average 12 year old that they can't get married and have kids would make them incredibly happy. Maybe that's just me. Anyway, to someone like Aang who was raised by monks, wouldn't the cncept of having a family seem odd? Maybe not, but there can't have been too many examples to follow at the temple. Maybe I'm wrong. I would love to know how Air Nomad society worked. I know it was to set up some Kataang, but beyond that I didn't see it hinted at. I'm happy to report.
What I did like though, was the changing to Firebending. Firebenders in the series can't create their own fire. They bend fire, in the same way that a waterbender bends water, but put a waterbender somewhere totally dry and they can't bend. In the film, most firebenders need there to be fire already before they can use it. Only real masters like Iroh cancreate fire, and it must be pretty rare, because when he does it the other firebenders run away. Sizin's comet, which as of he end of the film is three years away rather than a few months, will give all firebenders the ability to create their own fire. Interesting. I assume it also makes them more powerful, or the final battle between Aang and Ozai and between Zuko and Azula will be less impressive. But we'll see. If, that is, movies 2 and 3 get made. I hope they do, now.
I think Dev Patel made a great Zuko. I knew he would. Also, Zhao was wonderfully nasty. I was quite pleased to see him die (in a fairly unpleasent way, it has to be noted). The scene where Zhao invites Zuko and Iroh aboard his ship was perfect. It replaced a lot of other stuff, the Agni Kai between Zuko and Zhao for example, but it was very well done. Zhao's incredibly cruel comments, Zukos shame and rage manifesting in firebending the candle flame, Iroh stoping him, touching his hand. Them both leaving. Perfect. Worth the whole movie for that scene. Bit then I always have been a Zuko fangirl.
Finally though, why change the pronunciation of the names? Everone made a huge deal over the pronunciation of Aang's name, but Iroh as 'Ee-roh', Sokka as 'Soh-ka' for me those grated much more. I barely noticed Aang's name. Katara pronounced the first sylable of avatar in the same way, which made it sound like that's just how she speaks. 'oang is the oavatar' odd, but not that bad.
A few more things. Why was Yue in charge of the Nothern Water Tribe? What happened to her father? And that whole narrated 'Sokka and the princes quickly became friends' thing was lame. But the main thing I remembers to mention was the huge wave Aang created during the seige of the north. In the series, he edges with the ocean spirit after Zhao kills the moon spirit, and in a rage destroys most of the Fire Nation fleet, presumably killing lots of people and leaving Zuko and Iroh to float on a price of driftwood from a desstroyed ship for weeks intil they reach land. In the film, he 'shows them the power of water' by creating this massive wave, doing absolotely nothig with it and then sinking it back into the ocean. Effectively, he is demonstrating to the Fire Nation the power of water when he is controling it. Now he's going to merrily pop off to the Earth Kingdom leabingthe fleet intact to sake down the Ayer tribe city of they want to.
This does solve one problem I had with the series though. In season 3, before he fights Ozai, Aang spends an episode angsting about how he never kills and doesn't want to kill Ozai even though he is evil incarnate. That whole time, I couldn't help thinking about what he did to those ships nhe could I suppose, hide behind the excuse that the ocean sprit did it, but I don't buy it, aang was just as responsible. Not any more. So that's good for his conscious I guess. But the fleets main purpose may have been to capture Aang, but they still know how to get into the ice city and wreak havoc. So even though it won't happen becuase it didn't in the series, I still say Aang left the water tribe open to attack.