Open content model?

Open schema vs features/properties

soapAction='http://www.example.org/myAction'

(schema validation, easy to write, part of WSDL)

vs

<feature
   uri='http://www.w3.org/2003/06/soap/features/action/'
   required='true'/>
<property
   uri='http://www.w3.org/2003/06/soap/features/action/Action'>
   <value>http://www.example.org/myAction</value>
   <constraint>xs:anyURI</constraint>    
</property>

(extensible)