Code as data is interesting, but mostly orthogonal to S expressions. For example, Prolog code has the same property without S expressions, and more esoterically TeX (which is succinctly explained to a lisper as programming with defmacro but not defun).
Though it's just more powerful in Lisp precisely because the code is just lists in a language designed around working with lists.
So you can actually leverage this property in Lisp without the code becoming inscrutable for it, which in my experience doesn't usually happen in other languages.
