Unity3D? I have no idea how it's like working with it, but I imagine that it'd be easier than Java
Notch should give Minecraft a dedicated engine.
I still don't really understand, is it worlds and stuff saving on your account rather than your computer?
It's basically one bigass hard drive that holds everybody's stuff.
The only downsides would be that the amount of worlds you can have would have to be limited, and you couldn't share worlds without Notch adding a world sharing system.
Oh, and that it would have to load the world each time, which could get quite tedious.
If you ask me personally, I'd say that a cloud for Minecraft is something that shouldn't come around.