[underwater]

I like that the syntax for features is illustrative, so that the raw text representation doesn't end up as a mess of ugly tags. However it looks completely inflexible. It's a mishmash of special cases. How would I implement a new feature without breaking existing implementations? Or without having to write a new parser that in all likelihood will break on some subtle edge case?

At its core XML (if you ignore all the DTD, namespace and entity rubbish) is both simpler and more powerful than this. You have text, tags and attributes. What those tags and attributes mean is up to the application, but at the very least you can be sure that the document can always be reliably parsed into a form you can work with.