tl;dr version - it's mostly about the portability, shortcomings of Windows as OS, and accompanying software (VS, TFS, IIS, MSSQL etc) being bloated. Zero critics about the .NET framework itself.
* Git is now a first class VCS for use with Visual Studio
* .NET development no longer tied to Visual Studio thanks to things like VS Code or OmniSharp with Submline
* .NET development no longer tied to Windows, dev with a Mac or Linux using .NET core
.NET core hasn't seen an official release and there's still quite a bit of work to be done. Even with that, the overwhelming majority of .NET libraries don't work on OS X or Linux and it's going to take a significant amount of time to port just a small fraction of them.
.NET core might turn out to be a great cross platform solution but it's nowhere near that right now.
And that's why I left the .NET Framework because it kept reasserting itself and wanting to be more than it was: an implementation detail.