Seriously though just adding some chrome like a start button that invoked the start screen would go a long way IMO.
It'd be nice if the bottom left hotspot was used "just" for switching between the start menu and the desktop. Then, you sort of retain that start menu like feel for keyboard/mouse users.
But, it all depends on whether the desktop is supposed to be an app.
On the Windows Store App front adding a "close" x in the upper right corner when the mouse is detected and/or moved would be a nice crutch for those coming over from the desktop world still stuck on mouse & keyboard (and insistent on closing their apps).
Yeah...you shouldn't have to use a touch gesture to close stuff. You still need to be able to drag apps for side-by-side, however. Maybe just have a VS-style radial menu popup when you go to drag the window, with one of the icons being a close button and the remaining two being for side-by-side? Basically...you wouldn't have to move your mouse so much to do something simple like close.