Full stack to me is more about ownership and responsibility than pure expertise. He's the guy that can literally carry the entire thing on his back if he has to. Technically speaking, usually because he's the one that created it, by himself, made it work, made it scale, and made it make money without any help. In my case he becomes the guy in charge trying to figure out how to hire people to just little parts of his job. And finding out he now has to learn how to hire people, just like everything else...

:)