The Legend of Korra

For the Air Bending part: If I remember correctly, it was mentioned that you are taken away from your parents to become an Air Nomad. So by that information we can assume that they determine who is an Air Bender before making them an Air Nomad.

 Spoiler

As for the Metalbending Police....maybe not all of them are that good, however they also don't even Earthbend at all. They throw away half of their abilities. Toph was so great because she combined the two. Lin even kicked some major ass when combining the two in the last episode, and the only reason she lost was because she made that stupid blunder with the Lightning Rods. She was winning before then.

The reason she lifts her foot to do Sensing is because she has to open the metal casing first. It's probably a ton easier to do it when the foot is in the air rather than when your foot is already plastered into the ground.

Characters like the opposite Pro-Bending team likely would have been important characters had they not lost their bending so quickly. Even though that guy was a jackass (The leader of the winners), he was still a skilled bender and would likely have been a powerful ally should he have kept his bending. There's other people like that. Most characters who lost their bending never had time to develop into major characters.

On Airbending: "The "Avatar Extras" stated that all Air Nomads are airbenders. This would make the Nomads the only group with every individual being a bender. The creators also explain this is due to bending coming from spirituality."

 Spoiler

>Obviously.

>I'm fairly certain there's no need to raise her foot in preparation for some sort of death strike to just remove her "shoes." If anything, she does it to hit the ground for more vibrations. That is the opposite of what Toph did as she just felt all other vibrations rather than creating her own (except that one time she was in a metal box).

>Does that not apply to all minor characters in movies/books/shows? "They could've been a major player if only ________."
To be honest, I didn't even think whats-his-face of the opposing Pro-Bending team was that great. Because he cheated so much, it gave him the illusion of being superior. But really, I'm certain in a pure waterbending duel, Korra would've won.

On Toph: I'm sure there's all sorts of exceptions. Because Toph is blind, she has similar conditions to Badgermoles so that may have given her some sort of boost (?) to just learn it naturally. Again, Aang's family is probably the biggest "evidence" that bending is genetic since one of his children is a Waterbender, the other an Airbender, and the third a non-bender.