It's not just a piece of Gorilla Glass, it is uniquely shaped to be convex in the interior, for diffraction. Further, there is special internal porting and chamber technology that reduces resonance, while giving a larger area for bass standing waves to develop and increase perceived bass without having to mess with an already great driver. The headband seems more comfortable and durable as do their pads. From what I can remember, there are other design changes inside with the goal of being acoustically transparent as possible. Do I wish they'd cost less? Yes. Still, having headphones with the same quality of mids and highs as the open version, with the bass many listeners have waited for, that does not bleed into the mids or add noticeable distortion, boominess or other issues is an achievement. If it gives 95% or more of the open, wide sound of the HD800s, in a closed can, mission accomplished. Many studios will be happy about this.
I have heard the HD700, HD800, HD800s and the HD820. I like them in that order from most harsh highs and lacking bass, to smoothest highs with the best bass. It'd been a while since I've heard the HD800s before hearing the HD820 but the perceived difference in "stereo width" was negligible. Until one does a side by side comparison in a quiet room, we'll not know for sure & that's not really something you can test for, it's a matter of perception for the listener. I heard an exceptionally accurate sound field, wide on wide-panned things, where it needed to be. I had heard the closed Audio Technica that tried to maintain that and to me, in the quest for width in a closed can, the sound was weirdly distant and not in a good way. The Meze 99, to me, have really wide sound staging. Yet they have enough punch in the bass and mono areas for a truly Left, Center, Right experience. So wide can be done in closed, and the HD820 did it very well.