The Legend of Korra

There was an argument made that the ability to be able to bend is genetic; I would agree with that argument. Firstly, Aang's son Bumi is apparently a non-bender. Strange since his mother is a Waterbender and his father is an Airbender (it's noted that all Air Nomads could Airbend, however). There's also the fact that a Firebender can't become and Earthbender and so on. Sure you can learn the theory behind bending, but I imagine it requires a specific gene to actually execute bending.

 Spoiler

>Yeah... that one was just a pure /facepalm.

>Firstly, I would imagine that not all of the Metalbending Police can use seismic sense. Secondly, I also am certain that while Lin is a talented Earth/Metalbender, she didn't inherit all of her mother's prowess. The two occasions she has been shown using seismic sense she had to lift her foot and strike the ground. That is never shown for Toph. So she probably cannot sense as well as Toph, and that makes sense. Lin can see, Toph could not. Toph had to use seismic sense.

>So many major characters? As of right now only two: Tarrlok and Lin. The rest have been shoved aside/are not major. Nonetheless I do agree that it is probably temporary.

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.