I think it's more than that. As Carmack observes, you don't see in 3D. You see two 2D planes that your brain extracts a certain amount of depth information from. As a UI goes, you can't actually freely use that third dimension, because as soon as one element obscures another, either the front element is too opaque to see through, in which case the second might as well not be there, or the opacity is not 100% in which case it just gets confusing fast.
You've really got something more like 2.1d to work with with humans; intefaces need to take that into account, and maybe sometimes use that .1d depth dimension to provide light hinting, but not depend on it.
So you're not removing a dimension... you're acknowledging it doesn't exist. There isn't really a "third dimension" here to take advantage of. It may be visually impressive if something flies from far away to up close, but in interface terms, all that has happened is that the z-buffer values shifted, but the interface still has the same number of square radians available that it did before.
The depth information is a lot more like an additional color than an additional spatial dimension. UIs are better with color hinting available when used well, but there's not much fundamental difference between a color 2D interface and a monochrome 2D interface. Proof that it isn't much is that there are some people who use settings to set their phone to monochrome, and the result is at worst occasionally inconvenient, not crippling. Compare with trying to go from a 2D interface to a 1D interface, which would be utterly crippling and destroy a huge swathe of current UI paradigms.
To truly "see in 3D" would require a fourth-dimension perspective. A 4D person could use a 3D display arbitrarily, because they can freely see the entire 3D space, including seeing things inside opaque spheres, etc, just like we can look at a 2D display and see the inside of circles and boxes freely. They have access to all sorts of paradigms we don't, just like no 2D designer would ever come up with a "text box", even though it fits on the 2D plane, because no 2D user could see inside the text box to see what's in it.
(This is one of those cases where the mere act of typing it out improved my understanding of the topic. I didn't have the "depth information is really just another color" idea going in, but the more I think about it the more sense it makes, in terms of the amount of information that channel can carry. Just like color, it's not quite 0, but you can't really stuff all that much in there.)
True, almost any interface that overlaps helpful data in layers or stacks is usually terrible. Hell, I don't even like desktop computer GUIs that allow you to have stacks of windows. I'd rather see one thing at a time and cycle between them. Or have a picker view.
That said we do actually get quite a bit out of that ability to see depth. People who lose depth perception have quite a hard time adapting to the world. And our spatial understanding seems to go beyond our vision. That to me is where a 3D interface might be really powerful. Sometimes things which we struggle to decode in 2D are just intuitive in 3D like knots or the run of wires or pipes.
As I said elsewhere in this thread I think the 3D interfaces that are really going to be powerful haven't occurred to us yet. And I believe that what we'll find in time is that there are things which 3D interfaces are tremendously advantagous for and using anything else will feel like a hinderance. But those will be things for which 2D interfaces don't already do an amazing job.
Carmack already mentioned the existence of "true 3D" content, for which you get a 3D interface whether you like it or not, so to speak, so I didn't go into that.
But making everything 3D, because VR, is as silly as when the gaming industry made everything 3D, resulting in entire console libraries full of games that looked like shit even at the time, pushing 4 or 5 frames per second and having other incredible compromises, when the same consoles are monsters of 2D performance. As nice as it may be to have truly 3D content available in psuedo-real space, there's no reason to insist that when you want to set the shininess of a given pipe that you need a huge skueomorphic switch as big as an old car stick shift that you can visibly pull popping out of your UI or something when all you need is a slider. (If anything, I'd think minimalism in a VR environment is a good idea, both to contrast the other content and to prevent detracting from it.)
I think that's probably the kind of crap Carmack is complaining about. We've already been around the same loop a couple of times already, and 3D, albeit on 2D surfaces, was one of them, so it's fair to look to the past instances of such BS and maybe this time try to move along the curve a bit faster. I'd say that if we can get this nonsense out of the way faster rather than slower, we're more likely to get to the truly useful 3D stuff that doesn't exist yet. Otherwise we risk 3D interfaces becoming something like the Wiimote, which IMHO was actually a really useful tool that has become despised solely because it was badly misused by so many games, because motion controls. (Another example we've already been through.)
One other hypothetical use of a 3D interface is as a way to conceptualize "true N-dimensional" data. A 3D experience indeed doesn't help any rational conceptualize of a rational situation but it might, maybe, allow you to mobilize the unconscious reflexes humans have for dealing with regular 3d space. But all this might also a 90s cyberpunk fantasy.
I think what you've said is all correct. And its mimicked, to some degree, by how we build our physical lives. Shelves and desks are not so deep as to obscure what you need.
That said, I think an element that's missed is tangibility. In the real world, a stack of papers and folders are a valid way organize your own desk. You know where everything is and allowing you to tune your layout and grow muscle memory is very helpful. VR interfaces should let the user organize as they see fit. Interfaces should be tangible, movable, sizable, etc.
Maybe someone should build a cylindrical fridge with a curved door, so you could rotate the shelves to get to things diametrically across from your view.
It needn’t have structural pole in the middle. Could use roller bearings along the perimeter.
I think it would work better than an ultra-wide but shallow fridge.
Fridge depth is useful for storing large objects like a 24 pack of water. The trick is keeping them mostly empty otherwise food will rot before you eat it.
Bizarrely, counter-depth (aka depth that both looks better in kitchens and is shallow enough you won't forget about half the food in it) fridges are usually a fair bit more expensive, despite being smaller. Even for what's otherwise the same model.
I’m assuming you meant “yes we see 3d surfaces in 2d” to be consistent with the parent comment? Also, I’m impressed that dolphins have that ability. How does projecting them to each other work biologically? Do they somehow encode what they see as sound (less lossily than humans do)?
You've really got something more like 2.1d to work with with humans; intefaces need to take that into account, and maybe sometimes use that .1d depth dimension to provide light hinting, but not depend on it.
So you're not removing a dimension... you're acknowledging it doesn't exist. There isn't really a "third dimension" here to take advantage of. It may be visually impressive if something flies from far away to up close, but in interface terms, all that has happened is that the z-buffer values shifted, but the interface still has the same number of square radians available that it did before.
The depth information is a lot more like an additional color than an additional spatial dimension. UIs are better with color hinting available when used well, but there's not much fundamental difference between a color 2D interface and a monochrome 2D interface. Proof that it isn't much is that there are some people who use settings to set their phone to monochrome, and the result is at worst occasionally inconvenient, not crippling. Compare with trying to go from a 2D interface to a 1D interface, which would be utterly crippling and destroy a huge swathe of current UI paradigms.
To truly "see in 3D" would require a fourth-dimension perspective. A 4D person could use a 3D display arbitrarily, because they can freely see the entire 3D space, including seeing things inside opaque spheres, etc, just like we can look at a 2D display and see the inside of circles and boxes freely. They have access to all sorts of paradigms we don't, just like no 2D designer would ever come up with a "text box", even though it fits on the 2D plane, because no 2D user could see inside the text box to see what's in it.
(This is one of those cases where the mere act of typing it out improved my understanding of the topic. I didn't have the "depth information is really just another color" idea going in, but the more I think about it the more sense it makes, in terms of the amount of information that channel can carry. Just like color, it's not quite 0, but you can't really stuff all that much in there.)