This is a great article on the subject by Siegfried Linkwitz... jump to section 3 if you're impatient and 3.2.1 for his '5 requirements'
Image depth
Can anyone offer a technical explanation of how a stereo system recreates image depth? Why are some center images behind the speakers, and others in front of the speakers, for example.
Should there be any depth to a mono recording, or should the image be directly in line with the speakers?
Should there be any depth to a mono recording, or should the image be directly in line with the speakers?