I think that adding verifiability to the language would be great. MS Research has been working on Spec# for some time now, but it seems like it’s always behind the current C# release (for instance, the current version works against C# 2.0, but who wants to go back after using C# 3.0? Not me…).
Yes, in the current release you can try to apply some of the features introduced by Design by Contract, but you’ll only get a limits subset of what Spec# gives you (and you’ll only be able to do runtime verification – I’ve got a small framework that does this and I’ll probably blog about this on the future).
So, like many others, I would also love to have Spec# features merged into C# by default. Wouldn’t you love to have it on the next version of the language?