This is a very cool idea and I definitely see the problem that it solves, but it also has the downside of not being a metaphor for something physical.

It's a little strange to see these controls which are currently very anchored in the corner of the window to be moving and sliding around, especially when the view inside the window is scrolling off the screen.

I could see people running into trouble when they've somehow moved the window partially obscuring controls. But if the controls are off the screen, I think it reinforces the idea that this "box" is a window into an application. It doesn't resize unless you grab an edge. (1) I don't mind making the tradeoff towards beginners understanding the computer when there are keyboard shortcuts for advanced users.

(1) This is now worse in OS X from a metaphor perspective. Before you could only resize from the bottom-right where there was a gripper. Now you can resize from any outside edge and there are no grippers/indicators. http://arstechnica.com/apple/reviews/2011/07/mac-os-x-10-7.a...