S-Expression are in many regards the AST of a Lisp program. Macros create the high-level domain-specific semantics. If textual representation is important as well, you can write reader-macros. Lisp is really good at this sort of "meta-language programming."

It still seems to me that everyone, eventually, implements Lisp in one way or another (or at least some small part of it).

edit: grammar corrections