XML schema languages: Oh, the pain!
Oh, the pain of XML schemas (and even DTDs), I have often torn my hair over their perceived necessity and the impossibility of specifying the things I really care about, which is an understandable use and transmission of related information, while at the same time being forced to enforce a strict ordering on elements, which I don't see the point of.
In internal projects we have so far always opted to use example documents to specify XML document structures. This works very well, but is not powerful enough when specifying information models. And it may not be acceptable for external specifications.