I've used it quite a bit. I even like the namespacing bits. I find that XML composes elegantly in a way that the JSON and friends don't.

My one request would be to bring back to SGML-like closing tag abbreviation:

That is, instead of

    <foo><bar>qux</bar></foo>

we should be able to write

    <foo><bar>qux</></>

I think this one change would make XML more "palatable" for the JSON/YAML/TOML crowd.

SGML also has tag omission to make this even less verbose if desired. Or short references, which basically let you define arbitrary tokens SGML recognizes and replaces into something else, depending on the element context. These techniques in combination can be used to parse s-expr, CSV, markdown, and even some JSON, for example. Though personally I agree with others here that SGML is first and foremost a markup rather than config language.

The problem with tag omissions is that it requires that the parser have tag-specific information to properly build the AST. You can still do generic parsing with balanced </>.

Or even better:

  <foo/<bar/qux>>

(Although standard SGML inexplicably specifies a null-end-tag character of '/' rather than '>', so this won't work in stock parsers.)

This looks absolutely hideous. I don't see why this would make me want to switch from YAML or JSON.

Interestingly I find SGML-esque to be quite unpalatable. I get the feeling doing code reviews would be nightmarish.

It’s significantly worse than single char s expression for my eye, anyway.