> A data representation language must be concise enough, powerful enough, and flexible enough to make it possible to use it to define its own schema and transformations, without it being a painful experience, while also being simple enough that a single person can write a parser for it in a reasonable amount of time.

Seems like a standardized S-expression format would fit the bill.

You could even try to make it work with existing XML tools by specifying a way to generate XML SAX events from the S-expressions.