What I miss about UI development on Windows is the commitment to help me create applications that seamless blend in, and look like something that Microsoft themselves had created. The introduction of web technologies has allowed the visual language of the Windows experience to fragment. Not just because Microsoft hasn't kept older applications up to date (like the Control Panel), but also because the modern tooling isn't providing the libraries to match their style.

From memory, this first started during Vista, where 3rd party devs just weren't able to match the translucency of the new Aero UI. And the lack of articles being published by the MSDN group with titles like "Here's how you add this new feature to your app" with examples in C, C# and Visual Basic.