"Available Wednesday, Visual Studio Community 2013 is a free, fully featured edition of Visual Studio including full extensibility."
So, it sounds like this will replace the Express edition and let you install extensions like you can in the Pro version.
"Visual Studio 2015 and .NET 2015: build for any device - Built from the ground up with support for iOS, Android and Windows, Visual Studio 2015 Preview makes it easier for developers to build applications and services for any device, on any platform."
It almost sounds like you're going to be able to run VS2015 on different platforms, but I doubt it. Maybe you'll run the web version of VS2015 to develop from Mac/Linux?
"To further support cross-platform mobile development with .NET, as part of their strategic partnership, Microsoft and Xamarin announced a new streamlined experience for installing Xamarin from Visual Studio, as well as announced the addition of Visual Studio support to its free offering Xamarin Starter Edition — available later in the year. "
This is very interesting - .NET is going fully cross platform but they haven't bought Xamarin...are they planning on competing while keeping their frenemies close or something else?