On the server side, you can install whichever .NET versions you need and they will live together quite nicely. But you can only install one Java version, and updates randomly break things (that's probably not true in general, but that's how it seemed to me). And I can only remember a handful of issues that were caused by the .NET framework, we had constant reliability issues with Java.
Having lots of languages available is nice, but it's not a core requirement for a platform in my opinion. .NET has C# and F#, and they complement each other very well.
I haven't had any library issues with Java or .NET, so I can't comment on that.
My guess is that my problems with Java stem from lack of experience with it, and my success with .NET is because of my deep experience with it. So it seems likely that your perspective is for similar reasons.