From time to time I tell people that there are all sorts of interoperability problems and complexities lurking in W3C XML Schema for the unwary.
Sadly, the allure of the GUI tools and the "support" of the big vendors makes the argument difficult to make. Add the fact that a lot of people jumping into XML Schema use have little background in XML and thus tend not to understand the space well enough to even understand the expresssion of the problems, never mind the problems.
Anyway, it drives me nut becuase the problems are very, very real.
For those who need to hear it from the big names before they will believe it, I give you ACORD, BEA, BT, HL7, IBM, Microsoft, SAP, Sun.