Note for non-developers: "UWP" is the Universal Windows Platforms - the programming interface used to create applications compatible with Windows Store, Win10 and with a Metro/Modern. Previously Win32 and .NET (which was basically just a wrapper around Win32) were the available interfaces.
The main issue I have with UWP and the Windows Store is that they seem to focus more on creating business through a semi-monopoly rather than through simply having a superior product. The crazy focus on wanting a store sort of made sense when they were trying to keep Windows Phone alive and reduce the app shortage that was one reason they simply could not get a decent market share. But now that they've given up on grabbing a piece of the phone market - Microsoft seems hellbent on creating a revenue stream on their desktop OS that is similar to the mobile app stores.
That is simply not going to work. It is not that a store is useless or unwanted product - Steam, GoG, EA Origin, UPlay, etc. are evidence to the contrary. But the store has to compete on being a good product in itself. The Windows Store is not a very good product, and there doesn't seem to be much focus on improving the usability, pricing model, developer support, etc. UWP is Microsoft using the API itself as a vehicle to create store activity.
As long as win32 exists, this will not work. The UWP API is a poorly documented mess with some very opinionated design decisions - specifically the usage of a XAML-based paradigm. A good OS API should be much lower level, and the MVVM, MVC, MVP, or whatever UI programming paradigm is in vogue, should implemented by libraries on top of this API. UWP is also squarely aimed at "light-weight" apps - I mean something as simple as having multiple windows is borderline impossible with UWP. Restricting access to the file system is a problem for a lot of applications, even if it has some security benefits, as the chosen model is clunky and inflexible.
Microsoft used to be about giving developers incredibly good development tools - everything from the low-level win32 API to the rapid-application development VB and .NET environments was about empowering developers. A hack with very basic programming knowledge could create very usable line of business applications with VB, and the flexibility of win32 allowed pretty much anything to built with sufficient skill, even if the C-based API was clunky and old school.
UWP is too restrictive and opinionated for the low-level bitwizard, while also being too complicated and with poor visual tooling for the RAD hacks. It is really great for a the subset of programmers who either love MVVM/XAML or who have the programming skill and chutzpah to bend those technologies to their will. In my 20 year experience of programming, that's a very small group. The really poor documentation doesn't help. Browsing the UWP docs is weird experience - it reads like marketing material, more than knowledge base for developers. It is disorganized, messy, full of dead links and dated information. Most curious programmers are going to give up within 10 minutes and go back to .NET, win32 C++, JavaScript, etc.
If MS wants to make money from a Store, they need to build a great store. Better than Steam. A lot better, since Steam has an advantage in the huge games libraries gamers already have on Steam. They should have learned from the mobile market failure, that playing catch-up is hard.
At the same time, and completely unrelated, they need to keep their OS relevant to users by making sure developers have a good API and users a good experience. Otherwise Chromebooks, Macs, desktop Linux, the inevitable desktop Android and even SteamOS (both technically Linux I know), will start making a big dent in their business. And if they lose their dominant position on the desktop, that will probably mean the end of Microsoft as we know it.
It is downright puzzling why they are not spending more resources on making the Store a great product. I sadly suspect the organization is completely broken and horribly mismanaged. I do not think we will see anything great from Microsoft again unless they go through an existential crisis and massive reorganization.