My WCF 4.0+1 interoperability wishlist:
I wish Wcf would also have this. For exmple:
I have actually posted a series of posts titled "Cryptic WCF error messages" which tries to supply the most common cause for many errors.
In addition there are some lower level requirements that are not supported today. Not all are priority zero, but I have seen all of them in interoperability scenarios.
I'm pretty sure I have more somewhere but this should be enough for one version :)