JSON is an incredibly inefficient format for shareable data: it is annoying to write, unsafe to parse and it even comes with a lot of overhead (colons, quotes, brackets and the like).

I'd prefer s-expressions.

Have you ever seen s-expressions being used as data exchange format in the wild? (outside of the LISP/emacs world)

Not sure there is a decent standard spec and a good Java (say) parser around. Is there?

Here's a Java parser: https://github.com/bpsm/edn-java