Ooooohhhhh.... where, oh where to start on this one.....
Yeah, I think that's an nice summary.
This will probably go down as a textbook example of how to poorly adapt material from one medium to another because what came out here is a disjointed mess of a film that makes the last Harry Potter movie (Half Blood Prince), look like it was executed by Spielburg in his prime. What's wrong you ask?
1) In attempting to cram the entire ~8 hours worth of events presented in the first season into the "child friendly" runtime of 1 hour 40 minutes Studio's demand the film ends up being a bullet point version of the first season. There's no story flow, no time for character development, and absolutely no sense to be made of what's going on here. This, folks, is where you either decide to make more films, make longer films, or tell your own damned version of the story that gives you time to hit the high points and establish who these people are while skipping that side trip they took over here to do that thing... you know... that only some purist is going to demand you stick to. Sometimes your Tom Bombadil's need to get tossed at the shredder.
2) The film is far too serious. The thing about Avatar (the show), is that the characters were kids but they were dealing with things in the same way kids do. Aang spent most of the first season hiding his internal turmoil behind a veil of humour. Sokka is the kid who wants to be treated as an adult, having been thrust into it by all the adult males leaving for war, but is in deep over his head and still hasn't grown emotionally past the phase where a "yo mamma" joke is the height of humour. Katara is in the same position but has gone through the kinda mirror imaging of Sokka that TV + Society tends to throw teen girls out of where she's gonna be the "mature one", who takes everything seriously but still is a kid underneath.
The film throws all this out. Aang starts in serious mode and never really gets out beyond the 30 seconds he gets to pretend he's still part of a massive tribe at the Southern Temple. Sokka is locked into "leader" mode far too early in his development. Katara is whiny female protagonist. Whether this is a failure of script, director, or actors, is something you can't really do from this end but it just doesn't work either way; the charm and relatability of the characters got gutted is the result.
3)Too many characters introduced results in wasted screen time. One of the other things the TV show was wise to do was to treat the Fire Lord as someone who had palpable effect on the world, yet wasn't an active participant until it was time for him to be the threat. It allowed you, and them, to build to the end game while focusing on the immediate threats to Aang et all within the current season. M. Night's mistake is to throw this out, forcibly insert him into the first film and create a bunch of moustache twirling scenes for him while depreciating screen time for the characters that needed to be introduced and elements that should have been explained now; like what exactly the "Avatar mode" is. Zuko and Zhao were meant to be the threats in this film; the Fire Lord could've been introduced (and cast), much later.
4) Flip side: wrong Characters introduced when others were necessary. Instead of meeting Aangs predecessor Roku to learn who, and what, the Avatar is we end up stuck with a spirit dragon, presumably adapted from the one Roku rides, who does neither; instead just spitting out gobble for Aang to sort.
5) Show, don't tell. Oh, god, does this movie trip over that axiom. On multiple occasions we get these tedious monologues telling us whats going on for the scene that should be being acted out in front of us. Mind you, Katara droning on about how Sokka and the Northern Princess "became friends immediately", got the largest laugh of the film. Which would be great, if it were intentional...
The sad thing is you begin to get the feeling we get these monologues not to hit runtime or because the scenes weren't shot, they're often being acted out in front of you while she drones, but instead because they just turned out like crap.
6) The action of the series was dynamic and energetic.... The movie.... not so much.... In fact, multiple scenes are just the actors doing the same thing to different targets and there seems to have been very little thought given to how to shoot large action scenes within the bounds of what these characters are capable of doing.
7) The movie itself has more kid gloves than the series it was based on. It frequently pulls it's punches: perhaps best exemplified in the final act which I'll only say deviates entirely from Aang, in his anger at the murder of the moon spirit and neophyte skill with his abilities, going full Avatar and rampaging through the Fire Nation forces as a giant water elemental to sweep them from the city in favour of... something less Hulkish and far more Ghandi-esque....
All this said, I'm gonna make the following suggestion: these days $15 gets you a 1 hour 40 minute movie. $26 will likely buy you 8 hours of the first season. Spend the extra $11. You'll have much more fun.