Mono Project Archive
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.