The Mezzanine

Hm. I like what you have here - the triple stat nicely mirrors the physical and mental triple stats, and I'm assuming you're in a d20 system, so they would add bonuses to rolls the same way?

Also, here are some edge cases! I find it's often useful to test the weird stuff, because it'll show up a flaw in the system faster than running a hundred iterations of standard actions.

Can people voluntarily use less than their total stat? Say, Zuko wants to light a candle. Since he isn't feeling like melting the thing into a puddle, he wants to make a tiny, focused flame instead of a raging wall of fire.

Can Zuko use his scope at 1, and his full agility, to light it with barely a flicker of waste heat?

Iroh knows he's made someone very angry and is ready to channel lightning. How does he distribute his skill points to avoid being fried?

Toph wants to throw a pebble. Can she scale up the power behind a given pebble without scaling up the size of the thing she's tossing? Or, likewise, can she gently float a boulder in Sokka's general direction before laying it down - lots of mass, but not a lot of speed?

Do non-benders have null stats, zero stats, some other way of representing they're not benders?

When character-building, how does a non-bender maintain about the same level of buttkicking as a bender? Or do you just not have mixed groups? Or is Sokka's player (for example) just aware that, and OK with, the benders being stronger in many situations?

...

I've kicked around the idea of an AtlA RPG using Exalted as a base, considering that Exalted already has (in the form of the Terrestrials) elemental 'bending' in a way. Remove the wood element, scale their physical power back down to the human norm, and there you have it... even the Avatar could be covered by a tweak of the Eclipse-caste charm rules (and again, scaling back the physical power to the human norm so you don't end up with one Solar in a world full of heroic mortals.) There's even spirits already! But it seems like that's not the system you're going for. It'd be a pretty heavy modification, but I think anything would be. ^^