Niggles: Since you answer with a question, does that mean there is no positives? :P
MaximumBunny: My understanding of it is that it allows cross-compatibility between Windows devices without the need to recode something for each individual device (desktop/console/mobile/other). That on its own seems like a positive, right?
That depends how you look at it. Generally, it seems like a sure-fire way of making shitloads of useless apps. We're already seeing lots of straight PC "ports" of mobile games, where little to no effort has been expended to adapt the touchscreen interface or GUI size to the PC reality. If devs didn't even have to recompile the damned things, I expect this would only get worse.
The thing is, desktops, consoles, mobiles and tablets are not the same, and an app that is well suited for one is not necessarily well suited for another.
Now imagine one huge unified app store with a bazillion apps which work on any platform, but which were each designed with a specific one in mind, only you don't know which one that is. I'd say that is bad news for customers.