In defense of WSDL
There also is one important aspect that Clemens seems to have overlooked, which I will generically refer to as bindings. When doing SOAP using XML over HTTP, a URI and a SOAPAction are required. Non HTTP and non pointy-bracket serializations of the XML Infoset should also be supportable. A concrete example: a binary message sent over MQSeries may be a vital part of a flow of business process.