I could be completely wrong, but here's a theory. It's actually the presence of spurious reflections that creates the sense of three dimensionality of glossy paper.
Let me explain.
I have been working with the new Harman FBAL gloss paper. When you first look at prints they are wonderful, truly the closest to the old glossy dried matte we have seen. After a while though, you notice that the paper is so glossy (on my iPF 5000 anyway) that it's actually a little challenging to find a position in which to hold the paper without creating any reflections. This isn't a problem with the more pebbly papers like Silver Rag, Hahnemuhle Pearl, Canon Semi Gloss and so on.
I then noticed that if I hold the image perfectly still in my hand, other than deeper blacks, the image doesn't look significantly different from my matte paper images. Should I move my head slightly or move the paper even a small amount, the three dimensional look comes back to the image. I actually think that it's the reflections off the surface causing variation in the brightness in the tones in local areas which is creating that effect.
Of course, I'm talking subtle here - I'm assuming you have already placed the image in a position which minimizes reflections so that those remaining are barely visible - eg. faint reflections off your clothing as you hold the print - not glaring reflections of the lighting. I'm also talking the tiny movements which are a normal part of holding the print up to the right position, not a deliberate rocking back and forth.
Perhaps I'm completely off target here but see what you think. If I'm right, it has implications for the differences between hand held and rigid dry mounted prints, between in the hand and on the wall, and between bare and behind glass images.