For one reason or another it would be awesome to write an Operating System in X, where X is some specific language or environment with some specific property.  I'm really excited about the work from Microsoft Research about an OS written entirely in managed code:

But wouldn't it be awesome if there were an Operating System written in JavaScript?  Obviously all the low-level OS functions are already taken care of by the browser, but seriously.  JavaScript can do a lot of neat things and when you add AJAX to the mix the sky is the limit.

... but what's next?  What about richER client experiences.  What then?  Would having the ability to launch processes, start threads, managed a windowing layer help out?