It's a F#-to-JS compiler, a UI framework, and an RPC system all built into one. Script# is similar, for C#, but I don't think it got that far? MS did start "Project Volta" IIRC but then gave up on it. There's also FunScript: http://funscript.info/ but that seems more focused only on the JS generation part.
I don't know why Micrsoft doesn't buy these guys and deliver this as a core solution. I'd guess it's because it's not on C#, and MS marketing doesn't know what to do with F#. They want to pigeonhole it as "F# is a special language for like, scientists and stuff. Don't worry, it ain't got nothing on C#." And hey, with enough time and resources, C# may catch up on some of the things. With Rosyln, I think it'd be easier to make a C# WebSharper thing.
There are already a few approaches out there for compiling C# to JS, like my JSIL (http://jsil.org/) and Script#'s unofficial successor, Saltarelle (http://www.saltarelle-compiler.com/). I believe there is a new Roslyn-based C#->JS compiler in the works as well.
I'm not aware of anything that approaches WebSharper's level of integration, though. It's a really impressive piece of work.
I don't know why Micrsoft doesn't buy these guys and deliver this as a core solution. I'd guess it's because it's not on C#, and MS marketing doesn't know what to do with F#. They want to pigeonhole it as "F# is a special language for like, scientists and stuff. Don't worry, it ain't got nothing on C#." And hey, with enough time and resources, C# may catch up on some of the things. With Rosyln, I think it'd be easier to make a C# WebSharper thing.