Both of which have been in F# for years, and C# re-implemented them later, poorly. F#'s dynamic is extensible (and around since 2007? 2008? earlier?)). If you wanted to implement a "dynamo" object, go ahead. If you want loosely typed CSV-reader access, go ahead. (You get the ? and ?<- operators to define as you'd like.)

C#'s async comes from F#'s async (available since Oct 2007), which in F# is just a library. That's because F# supports workflows aka computation expressions aka warm fuzzy things. What F# does as a library feature, C# requires baking into the compiler.

So for 5 years now, MS has added these two features. They've still yet to deliver a REPL, or finish implementing type inference, or any such stuff. It looks like Anders Hejlsberg, the main C# designer, seems more interested in adding some basic features to JavaScript, lately. C# seems fairly abandoned, language-wise, to me. They seem to be content to have put Java in its place, and leave it at that.

P.S. C#'s a great language, and I'm not trying to insult the people that have done great work on it. As a popular language, it's one of the nicest ones out there IMO.