With Microsoft Windows 8.1, the Windows Store was accessible from the Desktop mode. But any apps distributed via the Windows Store had to be apps built to use the Modern UI, a style more commonly known by its defunct title "Metro". The prohibition on distributing pure native Desktop apps (e.g. the Paint and Calculator apps in Windows 8) via the Windows Store is being lifted in Windows 10.
Microsoft's Windows engineering team writes in a blog:
The Windows Store will also support more than just modern apps. It will add desktop apps, as well as other types of digital content. We will provide many different ways to pay for apps. And we'll provide an organization store within the public Windows Store, where an org can place their own curated list of public apps as well as specific line-of-business apps that their employees need.
The blog has since been pulled offline and is no longer available in Google web cache. It appears that Microsoft perhaps wasn't quite ready to let this cat out of the bag. But there's no reason yet to second guess that the above plans have changed.
"Other types of digital content" hints that Microsoft may expand to sell movies, TV episodes, and songs. That's not all that surprising (assuming that's what it means), given that Google's Play Store and Apple, Inc.'s (AAPL) iTunes/App Store/Mac App Store already do that.
If Microsoft can offer for PCs a unified marketplace à la Google's mobile device Play store, it will be a first in this space. Currently Apple does sell both apps and media, but its marketplace in OS X is split into two separate apps -- iTunes for music/video and Mac App Store for apps. That separation appears to persist in OS X "Yosemite" (v10.10), which is expected to release in finished form later this month.
On the flip side, the move could further outrage the likes of Valve and its quasi-CEO Gabe Newell, who were already ranting against the decision to include the Windows Store for Windows 8. Mr. Newell opined that Windows 8 was "a catastrophe for everybody". A major point of contention was his feeling that Microsoft was hurting third-party app stores like Valve's Steam. Some other third-party Windows app store proprietors echoed his sentiments.
With Windows 10, that outrage should grow as Microsoft further distinguishes the Windows Store as the place to get Windows apps of all kinds, including Desktop and Modern UI ("Metro") styled apps.