Question raised via Tim Bray: If you’re building an Operating System, what’s that first feature that you build, test, and deliver?.
Taken on its face, it's a question without a good answer. No customer is going to pay for part of an operating system when they can get a full version of Windows, OS X, Solaris, Linux, BSD or any of dozens of other off-the-shelf systems. One feature of an OS is worthless in today's world.
Really, the same question could be asked for any large, complex piece of software. Until there is a real customer willing to pay money, someone else (for example a product manager) has to take on the XP customer role. (Even after a product has real customers, their conflicting needs have to be reconciled somehow. That's part of the responsibility assigned to the XP customer role.)
So the answer remains, whatever feature the customer decides has the most business value. There's a whole book about what features to implement first to maximize financial returns (not just ROI).