This blog post from Simon Guest is a bit dated but still useful in my opinion. I can’t tell you how many times I run across vendors or trading partners who assert that they are providing a “web service” who violate one or more of these guidelines.
I would probably add the following:
5A. No Arbitrary CDATA-enclosed blobs of XML or proprietary text. I’ve seen this recently both in questions posted here on wMUsers and at one of my customers whose trading partner provided a “web service” that was not described by a WSDL and contained a large CDATA-enclosed blob of text that had to be parsed separately.
BTW, one of the commentors to Simon’s post suggested that his definitions were too narrow as they would exclude REpresentational State Transfer (REST) web services.