In an post titled The Perils of Premature Standardization: Attention Data and OPML, Dare Obasanjo writes:
I used be the program manager responsible for a number of XML technologies in the .NET Framework while I was on the XML team at Microsoft. The technology I spent the most time working with was the XML Schema Definition Language (XSD). After working with XSD for about three years, I came to the conclusion that XSD has held back the proliferation and advancement of XML technologies by about two or three years. The lack of adoption of web services technologies like SOAP and WSDL on the world wide web is primarily due to the complexity of XSD. The fact that XQuery has spent over 5 years in standards committees and has evolved to become a technology too complex for the average XML developer is also primarily the fault of XSD. This is because XSD is extremely complex and yet is rather inflexible with minimal functionality. [...]
Many people have commented on the problems of XML Schema, but rarely have I seen the negative impact of XSD stated so directly. This may be because intelligent commentators are polarized into WS-yay and WS-nay camps: The WS-yays have little reason to critique XSD (except when claiming that it doesn't matter because the tools will save us). The WS-nays have the whole WS stack as their target, so do not focus on relatively obscure technical issues in specific layers.
But I wonder, perhaps the great WS programme is doomed to failure, not because of the conceptual issues raised by the ReSTafarians, but because of this and other similar flaws in the layers comprising WS-*, leading to structural weakness in the stack as a whole.