I have to agree on everything that's been said about the characterization and world building in Avatar. Excepting the childlike fun I get from how cool bending is in how it's choreographed, it's what makes this show so interesting to me. I'm currently rewatching The Last Airbender and episodes like the ones dealing with Appa's kidnapping really work on a profound emotional level for all characters. And Korra even tops the previous show in terms of character development and complexity of the setting, at least in how densely it is presented.
Bending is supposed to be based on spiritual ideals and martial arts right? What prevents a non-bender from enjoying the same spiritual benefits by learning martial arts or meditating? The way that the benders have been reacting at the prospect of losing their super powers, it seems like there is a little more than spiritual attachment.
It's never really explained, but my interpretation is that bending is not just a function of someone's spirituality. You have to be born a bender, and learn the techniques associated with your element to use it [I'm not exactly sure how much spirituality is required to actually use it, but say Ozai certainly was powerful without being spiritual]. On the other hand, there are spiritual people who can't bend, like Piandao or Aang's guru.
I kind of get the impression that they are reacting the same way a child would if you were to take away some cherished toy. I get that bending is a great tool but I'm not seeing the spiritual value in shooting fire out of your hands while doing yoga or whatever.
Would Buddhists somehow be more enlightened if they could fly around?
Spirituality in the avatar universe seems to be about being in tune with yourself and the world around you. And for benders, bending is part of what they are. It's how they define themselves. So losing bending is not just like using a toy or a useful tool, it's like loosing a part of themselves. Sure, you can exist without it, but it's not really yourself anymore, so their spiritual calm is destroyed.
Imagine you're a master pianist, to whom playing the piano is part of his life. It's not just a nice hobby to you or a good job - you literally couldn't imagine to not play the piano in your life. And then your fingers become paralysed - everything you've been is totally destroyed.
Sure, you can live on otherwise unimpeded, and there are millions who never learned to play piano the way you did and live happy lives, but that's irrelevant to how you feel about yourself.
EDIT: And another question. What do the automobiles and those mechs run on? Rewatching the last episode, I didn't see any exhaust coming out of those mechs, and they had some sort of electrical generating ability.
That brings us to the question of generators again. If there's no exhaust, it supports electric motors instead of combustion engines, and that would imply there are generators too.