XML saves the world <?>

No universal data model

XML is built on an annotated hierarchical structure.

The hierarchy of values seems to be a good approximation to a wide range of application data models.  But it is not a universal data model:  not every application requirement is entirely captured in this way.  So designers must still be clear about the data modelling needs of their own application, and must be prepared to find creative ways to use XML to represent those needs.

I have seen cases where application designers have tried to use XPath, a language for identifying parts of an arbitrary XML document, as an application query language.  The results are not always edifying, and serve to illustrate that a pure generic XML selection technology may not be best-suited for an application's content-selection semantics.