I think it would be a bit clearer about the dragging and dropping to position the monitors, if there was no spacing between them (as it would be when all the content is stretched between them) and also I like how in GNOME the panel (so in your case wingpanel) in shown on the monitor previews
For the selected display an option to turn mirroring on or off, and then a drop down with the other monitors available for mirroring. When monitors are mirrored it could be displayed underneath their names that they mirror another, eg
What if you want to use absolute placement of monitors e.g. when I use my laptop and a second monitor, I want to make the laptop screen about 200 pixels lower than the monitor, so that windows and my pointer stay level when I drag them across the border. Currently the NVidia XOrg settings let me do this.
Which is exactly what I was thinking about. You, sir, are the master of minimalistic interfaces. Which, by the way, leads me to ask you something. If the application's icon has been dropped from the window, why not the application's title?