The Speed Bumps

Lots of options in Babel

Semantic problems - the Tower of Babel story The most obvious complaint about XML is its lack of a unified vocabulary, that XML gives people too much freedom. This is the accepted problem and potential of XML.
XML 1.0 options XML 1.0 allows non-validating parsers to ignore external resources.
XML 1.0 + Namespaces in XML While the W3C is pushing Namespaces hard, there isn't much consensus on details, and many developers and tools simply ignore this spec.
Internationalization (i18n) XML parsers are only required to support UTF-8 and UTF-16, while much of the world writes in other (perhaps legacy) encodings.
XML family problems XML has grown into a family of specifications, and there is not yet any implemented means of determining which parts of that family a given application supports.
Working outside the network Many XML documents and applications assume full connectivity to network contexts, which isn't always the case.