Which properties are optional depends on the protocol

If the SOAP protocol being used supports the SOAP request/reply MEP 
(which is true somewhat trivially right now), then [source],[reply 
endpoint], [fault endpoint], [message ID] and [relationship] should all 
be OPTIONAL.  They may make sense in some larger context, but they're 
not required to make request/reply happen.  The SOAP protocol is already 
providing this.

If the SOAP protocol being used does not support the SOAP request/reply 
MEP, but does support the (as yet hypothetical) SOAP one-way MEP, then 
at least some of the above will be required to make the MEP work (see 
previous message on orthogonality, though).

I tend to think much of the current debate over [reply endpoint] [fault 
endpoint], the anonymous EPR and so forth stems from trying to support 
both of these scenarios simultaneously in the core.