XML saves the world <?>

It's just a syntax

XML provides a uniform syntax that can be used by a wide variety of applications.

This uniformity of syntax paves the way for combining data from diverse applications in a single framework ("language mixing", see extensibility).

But it is only a syntax.  Application designers still need to master the other aspects of their applications.

XML is a language syntax framework without semantics. In the field of computer languages, syntax has been pretty well understood. It was nearly 50 years ago that Noam Chomsky first published a description of formal language [1], and the use of formal syntax descriptions and analysis techniques developed rapidly from that point. Most syntax description and analysis techniques in common use today were pretty well understood and documented by the early 1970s.

Today, programmers and information designers are still wrestling with the subtleties and intractabilities of semantics and processes, and XML alone provides us with no real help in those areas.  Trying to understand an application semantics purely in terms of its XML syntax is a losing game for all but the simplest of applications.  The choice of mnemonic names for XML elements and attributes goes a fair way to helping, but structural elements that don't fit the simple hierarchical model can be hard to discern.

[1] Chomsky, N., Three models for the description of language. IRE Transactions on Information Theory, 1956, IT-2(3), 113-124.