[joefkelley]

A typical generous estimate of human visual acuity is 0.3 arcminutes. As in, any pixel density beyond that is probably not visible to us. It's not an exact measurement since there's disagreement over how to properly measure it. But regardless, most estimates are in the 0.3-0.4 range.

Then simple trig tells us that we are maxed out when:

(distance to screen) * sin(0.3 arcminutes) = pixel size

The new iMac has a 27 inch diagonal, and 5874 pixel diagonal, so its pixel size is 0.004596 inches. Plug that in above, and you can see that you would have to be 53 inches from the screen to be maxing out your visual abilities. Anything closer (like normal) and you actually could still benefit from higher resolution.