Web Services Interoperability...
Here is a great article discussing exactly what "Web Services Interoperability"
means and the role of WS-I.
WebServices.Org - The Web Services Industry Portal - Interoperability: the Key to Web Service Quality
I was particularly taken with the line;
"Interoperability needs to happen at all levels of the Web services protocol stack for a Web service to be certified as "interoperable".
From my perspective there are still too many manual stages in the usage of Web Services, still too many places for human error to creap in. We are still a long way off of complete automation, and standards such as Web Services Metadata Exchange will help.
Looking even more forward into the future I can see a lot of applicability for the theories of Jaron Lanier, see this interview. I can see the WSDL and WS Metadata being the basis for the surfaces that he talks about, and UDDI could form the landscape on which the surfaces lie.
However that is a long way off, and in the mean time we need to work towards much better interoperability for simple web services.