"One of the main new features of ASP.NET 5 is unifying the programming model and combining MVC, Web API, and Web Pages in single framework called MVC 6.
In previous versions of ASP.NET (MVC 4, and MVC 5) there were overlapping in the features between MVC and Web API frameworks, but the concrete implementation for both frameworks was totally different, with ASP.NET 5 the merging between those different frameworks will make it easier to develop modern web applications/HTTP services and increase code reusability."
(and limited backward compatibility)
ScottGu's Blog - Introducing ASP.NET 5
Monday, February 23, 2015