[ericbb]

I think that M-expressions are quite interesting but I'll just make a few concrete points here.

> if we want (quote x) it looks as if we have to write: ... quote[x]

I think that you could just write X in that case. Uppercase text was for symbolic atoms.

Also, you didn't mention conditionals:

M-expressions:

    ff[x] = [atom[x] → x; T → ff[car[x]]]

S-expressions:

    (defun ff (x) (cond ((atom x) x) (T (ff (car x)))))

The semicolon and the infix arrow have the benefit of reducing the need for parentheses in that case. And you can avoid writing "defun" and "cond".