For human-readable document content, there's nothing wrong with plain old HTML, though formats like RST or markdown are somewhat easier to produce. (I save my blog documents in markdown format - they're just easier to write and update, and I don't have to futz around with JS-heavy WYSIWYG editors.)
However, these aren't adequate for structured data that falls outside the scope of a generic print document. For that, I would recommend YAML or JSON over XML, not only because they're human-readable but also because they're far less verbose.
I'm definitely fine with using data formats for things that really are just raw data (e.g. a series of records with specific fields), and I like JSON too.
I just hope that they don't try to chop up something that should be "just a document" into a pile of ugly marked-up containers that make it very hard to see what's even in the document anymore.
However, these aren't adequate for structured data that falls outside the scope of a generic print document. For that, I would recommend YAML or JSON over XML, not only because they're human-readable but also because they're far less verbose.