Rumor has it that the second gen butterfly mechanism inherited from the Pro makes this MacBook way more usable—so what's inside?

Comparing the space gray "new" keyboard to the rose gold MacBook of yesteryear, we can see:

The mechanical switch that detects the keystroke is a simpler rounder dome, not the fancy 'x' shape it once was.

The plastic butterfly mechanism also accommodates the new switch, swapping to a new, slightly thinner frame.

While not really a mechanical change, the control and option keys got some new ink. They now mark keyboard shortcuts rather than translating for PC users.