Likewise Async. The basic file IO and ADO.NET APIs can be wrapped up in awaitable structures but you pretty much have to do it yourself. For most purposes if you really want to make good use of C#'s cleverest language features you have to write wrapper classes around .NET to make it work.

They've also done a terrible job of making a first class Windows UI library for .NET; WinForms was always clunky and limited in the subset of MFC/GDI it exposed; WPF was too radical and didn't make sufficient use of the underlying native capabilities of Windows - and the WinRT story has been confused from day 1.