Magic? No, it's not magic, it's the control of chi. Not everyone can do it, but it's not magic.

[If Arthur wasn't so charming and nice, Jinora wouldn't be happy to hear airbending accused of being magic.]

It's about flexibility and constant movement... opportunity and defense. Firebending, earthbending, and waterbending are more offensive in nature, and I think that Korra doesn't know how to apply what she's learned about more offensive elements to airbending.