Just remember, TLoK was only written as a one season thing.
And as for the whole airbending thing, Korra knew the forms for airbending, meaning there was no reason for her not to be able to airbend, it just never clicked. Then there was that episode when she was in the match and realized that airbending isnt about direct confrontation but rather about defence, and redirection, alot like waterbending. Also her personality wasn't inline with an airbenders.
AND IIRC in TLA there was an episode that stated that bending was a state of mind, not a set of forms, it's why there are several styles of waterbending(swamp tribe), earthbending(Toph), firebending(Iroh, Aang's first firebending teacher)