I never really understood this part of the series either - Appa the Air Bisons' breed, for example, was said to have been the inspiriation that taught the Air Nomads how to Airbend, while the giant Anteaters and giant Moles are said to have taught the Earthbenders how to manipulate their particular element - I never got why mystical fish symbolizing the moon and the ocean somehow provided all the power for Waterbending. I don't think the movie was equipped to tackle that plot point - especially when it was resolved in a slapped on romantic scene that, while better than the groan-inducing one in "Attack of the Clones", was still very bland.

I actually thought the films' Sokka, Katara and Aang were completely different characters from the ones in the animated series. Aang and Sokka bonded over their mutual love for humor and games, for example. Every member of the cast just seemed too depressed to laugh.

Oh, and I have to make fun of this dialogue:
Aang: I need to find a spiritual place to meditate. Is there a spiritual place near here?
Princess: There is a spiritual place near here - in fact this city was built around it.

Pretty sure Aang could meditate anywhere - after all, he did it seemingly in every other scene in this movie.