The .NET CLI and CLR have always been great technology, but they've been closed. For many years Microsoft's approach was to support a broad range of 1st party languages on the CLR - COBOL, C#, J#, VB.NET, ASP.NET, and many more. The .NET CLR supported many more languages than the JVM for years. Microsoft could do this because they could marshal more smart engineers for a single project than any other company. The resulting success of "internally directed development" built an amazing software corporation to be emulated and feared.
Looking to the future: what would success look like?
To understand if open sourcing .NET is meaningful, we will need to look at activity over the next 12 months.
We should look to see if there are a number of new community-led language projects that show up for .NET - these would be observed through their usage of Roslyn (the .NET Compiler). Will a vibrant community show up to build Node.js for Roslyn? Erlang? LISP?
Another thing to look for will be whether there is a significant growth in use of .NET to target Android and iOS. With lagging market share for Windows on mobile devices, winning hearts and minds will have to show up in the use of .NET to target non-Windows devices.