To me, it was also the large side area on the 747 UD window seat where you can store stuff.
On the polaris seat, you have something similar with the space between the wall where you can store stuff but on the polaris seat, its a wall that blocks you when you are sleeping.
I think those higher walls on both sides is what gives that "coffin feel" versus being open. But I guess that is the price of privacy too. There is only so much those seat manufacturers can do with the limited width of the plane.