There are some open source components here, but I don't think the .NET platform as a whole is being moved to open source. The core compiler and runtime are open, but some of the most popular libraries and frameworks are still closed, it looks like.
There isn't much that hasn't been open sourced in the past few years: The runtime, standard library, C#/F#/VB compilers, ASP.NET, Entity Framework, Xamarin, PowerShell, Winforms, WPF are all open source or (in the case of WPF) being open sourced.
Roslyn is the compiler. There's not much of a debugger in it, is there? I mean, I guess the debugger will use Roslyn to parse watch expressions and evaluate things in the Immediate Window, but beyond that I think debugging is handled by something else.
