Amusingly, my brother hasn't seen the tv series and he found the plot to be really simple and is blabbergasped why others have a hard time getting it. Oh and his enjoyment to Golden Sun helped (again, amusingly) understand it.
I personally didn't mind the movie and liked it for mainly the scenery and costume work. Like my brother, I found the story to be rather simple to follow, the problem w/ the movie to me is two main things: pacing is a bit too quick and lack of fighting (considering from the....3 episodes I saw from the series, i'm rather shocked at practically little to none fight scenes, despite the fact it's based on a series of martial arts and elemental control). Pacing well...affected the whole move like a domino effect, acting is good (I noticed enthusiasm on mainly the main characters, but pacing killed it), scenery is good and adventure is good (but pacing killed it). Pacing to me added an odd effect to the whole movie, making it worse.
For once, this movie would probably have been slightly saved to be at least above average by being longer. Which then would help space out the story, rather than throwing down summarized chunks at you and let the audience get a better build up and alas, FIGHT SCENES, proper and faster ones.
That's about it, I hope I typed comprehensive enough ^^'