Always thought that would've been a good premise for a season taking place after Korra. Deeper look into the sub elements to answer those questions.
Sandbending seems to have a lot more flow to it that normal earthbending, like Toph is a novice in ATLA presumably she gets stronger at it, but i see it as adaptability, Metal Bending strikes me as more set in your ways, unyielding to current events, the epitome of earthbending in fact, and lava bending is passion in what you believe in, more heart than brain.
So you have 3 precepts in earthbending that categorizes 3 elements, Unyielding Earthbender, Passionate Firebender (or emotional, look at azula's rage) and the Adabtable waterbender, with none of the others do you see such a range.
Firebenders, the ability to create lightning is splitting the energy within them and directing it out, but to redirect it like zuko and Iroh, requires flexibility and a calmness that isn't really inherent to a firebender.
Waterbenders, can freeze things up and even manipulate the blood within someone which you can think of as cooly illogical and emotional.
Airbenders, can learn flight and even spirit projectment via their detachment to worldly affairs. They are flexible in combat thanks to their training and very adabtable which makes me think waterbender, a powerful airbender can quite literally halt a target and even slay somone by it, showing determination, ruthlessness and unyielding will but you don't seem many airbenders becoming passionate or from the heart.