Apart from the new .NET Reference Source experience, another thing that Microsoft has improved (well, has made actually work) is the Debugging .NET Framework feature of Visual Studio, which until recently was basically broken. Now, that feature doesn’t break with changes in the .NET Framework binaries due to security patches or hotfixes. Although Microsoft’s post mentions that it works with Microsoft .NET 4.5.1, it actually works with Microsoft .NET 4.0 and Microsoft .NET 4.5, at least if you have Microsoft .NET 4.5.1 installed (since 4.5 and 4.5.1 are in-place .NET Framework updates sharing the same CLR 4.0).
It definitely doesn’t work with .NET Framework 2.0, which, alas, is what I am still using in my MZ-Tools add-in (to support Visual Studio 2005/2008). But I guess that I can switch temporarily to .NET Framework 4.0 when I need to debug some .NET Framework assembly (just yesterday I needed to).
Of course, you can’t debug Visual Studio assemblies with that feature because their symbols are not available, but you can do it with .NET Reflector, as I explained in my article and in their blog as a guest post.