I do most of my development in C#/.NET, and I have to say you're right. I think their next step should be towards getting all of their desktop programming into the .NET API; they didn't have time to do it in Windows 7, but perhaps for Windows "8". I'm talking about things like drivers (which can be programmed using the now-free .NET Micro Framework) and lower-level APIs (like writing a screensaver...it should just be a class implementing IScreensaver or whatever, and not written in C any longer).
Getting everything (or as much as possible) switched over to the .NET framework in Windows would also help a great deal as far as mitigating security risks as well.
Not to miss the point of your message, but screensavers can be programmed in any language. I made one in .NET. The only constraint is the program needs to accept some command line parameters.
But I agree, it should be an abstract class/interface.
Having the ability to do more system programming on .NET (without having to P/Invoke all over the place) would (should?) make a lot of it significantly easier and faster. Also, the security issue is not to be overlooked -- with managed drivers and lower-level libraries, there'd be a much lower chance of fatal security bugs (e.g. buffer overflows). Also, writing this code in .NET allows developers to take advantage of a great number of new testing tools like Pex and Code Contracts (for fuzzing and static analysis, respectively).
Also, .NET 4.0 includes some additions for doing parallel processing to help developers take advantage of multi-core processors. I've been playing around with it for about a month now (I'm writing an open-source managed numerics library in C#) and the new parallel stuff is really quite good.
I would love to see .NET better integrated into the actual operating system. I agree completely with you.
Also, it's worthwhile to mention that .NET is available and usable freely (albeit limited features in VS, but not THAT limited), and let's not forget that start ups can get full access to plenty of the software through http://www.microsoftstartupzone.com/BizSpark/Pages/FAQ.aspx
I like the direction Microsoft is taking the company, I hope it continues.
Getting everything (or as much as possible) switched over to the .NET framework in Windows would also help a great deal as far as mitigating security risks as well.