Interesting, particularly about Common Lisp (does Scheme have this problem?). I wasn't for a moment trying to claim that "guarantees over expressivity" is never a good idea, I'm more trying to isolate the general principle so we can think about it - saying that it's always a good idea is probably just as stupid as saying it's never a good idea. As totherme says above, it's about finding the sweet spots.

And welcome! How did you find me?