A matte surface introduces Total Internal Reflection. Which means some of the light is reflected back into the masking of the display and absorbed. Almost certainly, some of this reflected light will be guided across the front laminate of the panel and contaminate neighboring sub-pixels.
A matte surface will also diffuse the light on exiting the panel. This can soften the edges at the mask boundaries when a matte bump extends over that boundary relative to the viewer.
A matte surface attempts to diffuse reflected light from the room over as wide a viewing area as possible. This reduces the number of resolved reflected light waves impacting your eye as compared to a specular surface. This is the only advantage to a matte display in terms of image quality. As far as I know. But typical anti-reflective displays (Glossy) make use of surface treatments to reduce reflections. These treatments work by pushing the phase of the reflected light into roughly the opposite phase of that same wave entering which cancels out on return. I don't think the glass of the iMac makes use of these treatments, however.
My guess is that people claim better colors with a matte display because the diffusive display will drop out some of the punch in the colors. A good thing or not seems debatable. Could be more of an issue of what we are used to. I am curious what the theory is here.
In this case, the same physics behind the improvement in black levels, also improves all colors and intensities. This is not always the case with black level technologies. Anti-reflective screens should, in terms of the physics, be a much cleaner image. The matte surface has always been a known trade off with manufacturers. I think apple may have fumbled by not explaining that placing glass over a glossy display negates what is actually possible in terms of reflection reduction. May have confused consumers into believing that glossy means reflective like glass. When it should mean much higher image quality with a slight compromise on resolved reflections (not reflections overall). That is to say, a glossy screen can actually be less disturbed by environmental light pollution than a matted display. This is because a matte display will diffuse reflected light that would have never been an issue toward your eye.
Sorry for the long post, but I just wanted to share that there is a clear benefit to glossy displays in terms of the physics involved. When implemented properly, it shouldn't even be a matter of taste.
Mark
it does look better when viewing video