Summary of my massive post because screw this forum:

Preventing bending crossovers:
- Water benders are bending some part or only the oxygen-hydrogen combination
- Air benders are bending nitrogen due to the ~78% atmospheric concentration and must have a conductive connection to the material (thus no evacuating closed containers - must go somewhere - rapid diffusion of gases generates the necessity of vortices for vacuum generation rather than just removing the air from a system - does allow for evacuation through a single opening)
- Fire benders are manipulating thermodynamics and acting as conduits (thus their requirement of bodily involvement - consistent conductive contact is the requirement for their bending, thus air bending can still quickly disable fire bending but it's through preventing long term contact rather than generating a vacuum)
- Earth benders are manipulating an elemental set excluding - iron, gold, silver, (all metals), and carbon (due to no bending of humans known or mentioned)

Density based ranged requirements:
- Fire - requires a bodily conduit - therefore almost zero range outside of creating flames due to combustion as well as lightning due to temperature differences and subsequent ionization. Almost zero defensive capabilities.
- Water - liquid concentration allows for "remote" manipulation due to trace hydrogen and oxygen in atmosphere and subsequent concentration of hydrogen and oxygen in liquid state water (due to the density properties of ice this also causes reduced range when bending ice and I think makes it consistent)
- Air - gaseous state of nitrogen causes limited initial range nearly to the degree of fire bending and therefore an ideal defensive style
- Earth - solid state of materials in question causes high range, conductivity requirement limits to in contact with earth. Due to mass and density allows for highest kinetic energy attacks and thus the aggressive style.

Economics:
- Fire has massive energy surplus due to state changes. Also Korra age universe is stupid for using lightning bending rather than steam and a turbine to generate energy (expanding gases due to state change being the most effective energy generation system). Additional metallurgy is ridiculous due to the ability to purify samples through available points of state change (more precise temperature control, better purity - which does beg the question of how anyone is going to bend their metal).
- Water has reasonable energy available due to hydroelectric power and turbines.
- Air has reasonable energy available due to turbines and manipulation of available atmospheric nitrogen.
- Earth has massive raw material surplus due to superior mining and metal purification through removal of the "impure" materials in metals that they can bend.

So long as the turbine gets invented and related systems. Everything is ridiculous.

Non benders are initially second class due to being effectively useless until the creation of technical schools to train them during the time benders are training their bending. Creation of finished products give them subsequent wealth to slightly compete with the bender monopoly on both energy and raw materials.

Avatar universe is effectively post-scarcity and scientific innovation should be extremely rapid. More surprising that they're not an advanced space faring civilization (outside of the requirements of actual innovation). And the difficulties imposed by the singular system they cannot appear to manipulate somehow (vacuum). (edit) unless of course religion is screwing up all of this

Korra conglomerate nation allows for ridiculous bending combinations - manipulation of state varied elements and subsequent abilities. And of course - so does the Avatar.