All the worst parts combined
ZDNet claims that Microsoft is working on an update to unify the Xbox One, Windows 8, and Windows Phone. This update, code-named Threshold, could be coming out in Spring 2015, and it will offer a common app store for all three devices.
This information is completely based information from unnamed sources on ZDNet, and Microsoft has declined to comment to them on "rumors and speculation'. It seems like a solid rumor to me, and sounds a lot like the ecosystem that Apple has in place. Of course, I don't have faith in Microsoft to implement this in a way that won't be obtuse, awkward, and cumbersome.