WSDL sucks. The whole WS-* protocol stack sucks. There, I said it.
It hurts, a pain in the but. Difficult to write and hard to debug. I cannot think of another technology that have wasted more of my time and I have yet to find one person that can give me a clear and concise explanation exactly how to to use it. Allegedly one of the most overengineered technology in the history of computers.
Doing wrong is easy, doing right is hard.
Its bloated. Tooling support is poor and hide necessary complexity. Interoperability is hard. Caching is not an option. Noncompliant with traditional web technology. The wire format is insanely verbose. Backward and forward compatibility is a nightmare. You have to read endless piles of specifications to design anything sophisticated; even sometimes bend over backwards to do the simplest of things. All your are left with is a pile crap to maintain in the end.
WTF? Where is KISS and productivity to be found in this mess?
System integration should not be this hard.