I've been on the "insider preview" for a few weeks now, and I must say that I think Windows 10 is an absolutely amazing OS. Definitely the best Windows I've ever used. Also the best desktop OS I've ever used, but I hope you'll believe me about the "best Windows" even if you can't imagine why a developer would possible want to run something other than $YOUR_FAVOURITE_UNIX.
If Windows 10 is as well-received as I expect it will, this might really impact Microsoft's position on phones as well. A core feature of Windows 10 is an app store that doesn't suck, with apps that can easily be used on devices that don't resemble tablets. Windows 8 really had this wrong, and 10 fixes it. I think the Windows 10 store might actually get used.
I've seen a glimpse of that future with the mobile app of Dutch weather site "Buienradar". They made a new Windows Phone app which totally rocks, to replace an old extremely crappy one. I didn't understand why they invested in an app ecosystem that is so clearly on the way down, until I found the exact same app in the Windows 10 store - just larger and with more info on the same screen. But it's very obviously the same codebase. My guess is they actually wanted to make a Windows desktop app, but adding phone support was such a minor extra investment (because of MS's "Universal app" thing) that they did it, despite the abysmal market size of Windows Phone in the Netherlands.