This is the first Mono release that contains code from Microsoft's open sourced .NET code. We are only getting started with this work. We are swiftly moving ahead in mono/master much more code that is being replaced and ported.
This version also is the first one to ship with C# 6.0 enabled by default. Learn all about C# 6.0 in only eight minutes on this presentation.
The release notes will tell you more.