.NET is a pretty strong contender and provides a lot of the benefits of Java, I agree. C# was ahead of Java language-wise for many years, though Java is catching up, and there are elements of the CLR and model that I'd prefer over Java, such as properties. I also recall the .NET assembly model being clearer than Java equivalents like classloader. await's pretty nice. It's all been focused around Windows though for years. (Random question: Do the concurrency concepts all work out on a platform with no IO completion ports? Is there a high quality .NET implementation using epoll?)

The open source community isn't there around .NET the way that it is around Java, however. Java's libraries are a big part of what make it so effective. Perhaps Microsoft's recent change in stance and decision to open source a lot of the toolchain will influence that, but it will be playing catchup for years. I see .NET as being an engineering coup from Microsoft with intrinsic value comparable to Java, while Java has that plus a large community and better cross-platform support.

When I work in Java, the wide availability of libraries and frameworks really helps. Take a look at the number of Apache projects written in Java: https://projects-old.apache.org/indexes/language.html#Java - many of those are very high quality and useful, and those are just the Apache ones.