I'm curious... how would you have developed a schema for S-Expressions without it ending up just as verbose? What do you lose? End tags?
http://www.agentsheets.com/lisp/XMLisp/ as an example of what XML in lisp would look like, and I can't say it's much of an improvement. Further, end tags make human debugging much easier.
There are many possibilities. SXML is the one I use.
You need to use a semi-structured editor, like paredit on emacs, to get all of the advantages, though. Otherwise as you mention you'll be grovelling for the closing paren, when the machine could have maintained the balance from the beginning.
http://www.agentsheets.com/lisp/XMLisp/ as an example of what XML in lisp would look like, and I can't say it's much of an improvement. Further, end tags make human debugging much easier.