That's as much as you get without spoilers. From here on in, I'll assume that you have seen either the TV series or the movie. If not, go watch the TV series, and if you love it, see the movie, but don't expect a lot.
First off, let me be clear: I didn't think it was a great film. Casting was poor (though, to be fair, finding children that can carry an entire and relatively heavy movie is nigh impossible) except for Dev Patel and some supporting characters. The boy cast as Aang was acceptable, but not perfect. Part of my problem with him, though, was the writing, and I'll get to that. There was also a rush to tell the entire story of the first season. Frankly, this is where the movie made its largest blunder. If they had done a 2-movie set with the first and last half of the first season, then I think we could have seen a really great pair of films, but as it was we introduced and rushed past many interesting characters from the first season, just to get to the Northern Water Tribe. This had to be done, because Sokka's love is introduced and killed there... an arc which requires at least some time to explain and create an emotional resonance for. As it was, her death still felt artificial and rushed.
So, what did I like about the film? Clearly M. Night had a deep respect for the visual pallet of the series. He kept an awful lot of sets from the first season and they look beautiful. The core story is all there, and though there were some subtle changes, I mostly liked how it all played out. No one's back story was really broken, so much as just bent in places. Little bits of humor were much appreciated, especially Sokka's, "I always get wet!" Appa and Momo have sadly tiny parts, but what little we see of them is as fun as ever.
M. Night's writing has to be a major topic, here, though. He just turned what could be argued is the best children's drama of the past decade into a barely passable live-action film. It's not that he's a bad writer, but I think that he gets far too absorbed in certain aspects of a story or film, and he's just not that director that can write and direct his own work. This is made clear as he repeatedly has characters repeating themselves or consoling each other with platitudes that fall short of the dialog he was starting with. He has problems with pacing, structure and dialogue which simply cannot be ignored. Next time, Mr. Shyamalan, can I humbly suggest you get yourself a script doctor? Ask Joss Whedon (whose script doctoring is legendary). At the very least, he can probably point you at someone well worth the trouble.
Anyway, see the film and expect nothing. It's a fun popcorn movie and if you can get past the rushed storyline and late introductions of major characters, it will be worth seeing.