Oh yeah, I started writting something about that earlier.
Benders vs Nonbenders:
The fire nation revered their benders as soldiers, air nomads were all benders, water tribe were given positions of respect seemingly religiously, but the earth kingdom was either "public servant drones" or "just that kid over there who moves dirt".
Each nation had a perspective on their own benders, and at least with the firebenders were seen with a lot of distrust because the 100 years of war, when all those cultures clashed and grew in one place for the first time, and the benders gravitated to eachother, gained power as their talents were more suited to entretainment and industrialization, the main pilars of the city, and the underside being the most notorious and dangerous of the criminals, people started to notice "It is kind of unfair that my new neighbor Jed can move earth with his mind and make a fool of my walls and I can't"
Before in towns and stuff a new bender kid was "the daughter of Zed and Ayla" or something, the sense of community keept those things at bay, but a stranger with superpowers? That's a menace.
That's my take on it anyways.
I liked what you wrote here
–- Update From New Post Merge ---
I chalk up the discontent with the benders as being due to the rise of industrialization and capitalism, and the onset of world peace. Industrialization gave benders an advantage, as well as certain cultural phenomena that developed in peace (namely pro bending). World peace eliminated the military reliance on benders and capitalism turned benders and non-benders into direct competitors for status and power, so non-benders had no reason to respect benders and all the reason to resent them. It was easy to turn lower class non-benders against the benders who had more access to the means of advancement. With time, the differences could still easily develop into class differences. I imagine the end result would look an awful lot like Zootopia's predator/prey thing.
I'd really love to have seen that plot extended over four seasons, with Amon actually being a non-bender, but unfortunately the first and second season suffered due to Nick being Nick.
I like this too!