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.