"An interface over an underlying process" is only one view you could take of WSDL - one that'd be encouraged by ideas like BPEL, I'd imagine. You could equally take an RPC view of the world (in the old CORBA sense). In this case, a WSDL can simply be thought of as defining an interface - like the
Java interface construct. This should let you apply more familiar design principles to deciding what goes in that interface.
I think of a WSDL as a facade to an application - this takes an RPC-view of Web services, which isn't strictly necessary, but IMO applies to most current uses of Web services. Like a facade, a WSDL (in theory) provides a clean interface to a system, hiding the complexity of implementation. This is the view I use when defining a WSDL.
-Tim