Bad red, bad! No touching enchantments! How about the Pillar of Flame/Annihilating Fire/Anger of the Gods "destroyed permanents are exiled instead" rider for the steam-powered form?

Fun fact, the steam powered version of this card isn't red. In fact, you CAN'T spend red to cast it.

I know, but it still rubs the color pie just wrong enough to be annoying to me.

How do you feel when a red deck plays All Is Dust? because they're basically the same thing.

Also, I think this should be a sorcery.

I'm fine with a strictly-colourless Akroma's Vengeance. Red's had Apocalypse and Worldfire in the past. I definitely agree this should be a sorcery though.

Circeus, "colorless" has its own slice of the pie that, in a way, constantly violates the rest of the color pie. Look at Lux Cannon, Brittle Effigy, and yes, All is Dust. Those technically break the pie as well.
By the way, Steam Powered might read easier as "If no colored mana was spent to cast ~, [effect]." I could easily see people thinking that they could cast this for and get the effect while still spending colored mana, because they would confuse colorless and generic mana.

True, but that wording takes the emphasis off the colorless mana, which is the important part. Yes, they mean the same thing, but the current wording, by specifically mentioning colorless mana, makes it more obvious to players that colorless is the theme, rather than leaving it up to them to make that connection.

That is true. I suppose you just have to decide where you'd rather have the potential confusion lie.