Well, it would seem that like her predecessor, Korra uses her naturally opposing bending art to a great extent.
What I mean with that is quite obvious. Aang was born an Airbender, an Airbending prodigy to be exact. This meant that his natural opposite was Earthbending, as the key to Airbending was constant movement and thinking freely, and the key to Earthbending was being unmovable and facing it head-on (literally). So of course he had trouble mastering Earthbending at first, but once he got it he became a very dedicated Earthbender. Throughout ATLA, Aang’s second most used element is Earth. Air and Earth are the elements he uses the most.
I see something similar with Korra. The only elements we’ve seen her bend is Water and Fire, so assumingly they are her most used elements. She was born a Waterbender, so Firebending was her natural opposite. The key to Waterbending is soft movements and flow of energy, the key to Firebending is fierce movements and power of energy. Firebending also requires much restraint and control, cause unlike Water you can’t just play around with Fire. So one can assume that she had trouble mastering Firebending at first, but once she got it became very dedicated to the bending art. So like her predecessor, her two most used elements are her native element and her opposing element.
Some fans (or rather many fans) have assumed differently. That Fire is in fact her go-to element, the one she uses the most, the one she’s truly mastered. And considering that she’s hot-headed and willing to fight, she had no problem mastering Firebending. The trailer and that sketch of her as a child support that (and I don’t doubt that it’s probably one of her favorite elements and easiest to use and master), but then again she is Waterbending in the poster and on other promotional art. Considering she’s born a Waterbender it’s likely one of her most used bending arts, maybe even her most used. In fact, in the first image we saw of her bending (your icon) she bends Water and Fire at the same time, which further supports my theory.
You have to really look at the two Avatars personalities to get a sense of why it’s like this. Aang is a pacifist, so he’s more defensive in his fighting style than offensive. Since Earth can work well as a shielding element, can be used to much more than fighting, and is readily available it’s no wonder he uses it to a great extent. Since Air is also readily available and his native element, he uses it to a great extent as well. But just because he uses it much doesn’t mean he’s a master at it, cause in ‘Sozin’s Comet part 1′ Toph said that his Earthbending still required some work.
Korra is hot-headed and not afraid to pick a fight, so her fighting style is more offensive. Firebending is almost entirely offensive, a very useful element in a fight, and it’s fueled by strong emotions such as willpower and anger. Since it’s source comes from within it’s an element that you always carry inside of you, so it’s readily available, so there is no wonder she uses it to a great extent. Due to the fact that Water is not always readily available as you need an external source to bend, it can explain why she doesn’t use it as much in the trailer when she fights. But when it’s available she probably uses it with great skill and prowess, perhaps not always to fight, I mean you have to admit her waterbending routine that we saw in the trailer is pretty impressive.
Due to her heritage and personality I believe that Water and Fire are her two most used elements, and the two that where the easiest to master. But since Fire is her natural opposite I still think it was challenging for her at first, maybe she had trouble with the restraint part like Aang did.
But I’m open to ideas