> But if I want something that is able to express data structures customized by myself, usually with hierarchical data that can be verified for validity and syntax (XML Schemas or old-school DTD), what other options are there?
He actually did address my question in a way: "[...] XML has prevented the adoption of something better (because it was "good enough")."
Which IMO is a sensible way looking at it. I too think XML is not perfect but if all the other stuff we're stuck with currently would be as good enough as XML, IT would be a place with less WTFs all around. ;-)
