Was Looking Glass more than just GUI effects? I have never found just graphic effects very innovative when it comes to a desktop environment, it’s not like it makes me more productive or shows me the information better. There are probably side-benefits of this like tuning a graphics stack that might be innovative.
Over the last 14 years my desktop environment has gotten more and more minimalistic to the point where I’ve gone from OS X to running i3 with no frills. I love it. Not having to drag windows around with my mouse is a step up for me.
Looking glass was really the first 3d window manager worth using. Like, you could annotate the back of windows and things. But, really… I’m not sure why we need that when 2d works just fine.
Another thing I remember about Looking Glass was a music player whose UI was a 3D stack of CDs. In a world of such 3D interfaces, I can imagine a need for a 3D analogue of a window manager. I think the major reasons to avoid it at the moment are our use of 2D pointing devices and screens (reducing 3D to basically a trick) and the prevalence of 2D, mostly rectangular UIs (making most 3D capabilities, like viewing angle, pretty useless).
Perhaps one reason to avoid 3D UIs is that in a 3D world, there’s always a “back side” and an “inside” which we can’t see, regardless of angle. That’s not the case for 2D UIs in a 3D world, or presumably for 3D UIs in a 4D world. Flatland comes to mind here :)