The good thing about Microsoft is that they're willing to take huge risks and when they fail, they're willing to change their minds and go backward in order to move forward.
Metro is an interesting flat design and it can be used as Microsoft's new UI guideline for desktop apps, now that it no longer mandated to be a full screen app.
Windows 8.1, in my opinion based on using OS X and Windows daily, is a much better OS at resources and responsiveness than OS X, but it is not entirely usable with its Metro design and so on. OS X is still better but if Microsoft does this right with the new direction for Metro on Windows 10, OS X might lose that battle and W10 could become the better overall OS. With that in mind, MS has said a lot of things and failed, so we just have to wait until the final release of W10 to see.
I can definitely say I'm not happy with what Apple is doing with Yosemite, the lightweight/responsiveness feeling is completely gone, it's not responsive as my favorite SL, and it is just eating up too much resources for no reasons.