I've seen people use "3D" to describe a mixture of many things, usually two or more of the following
  • Exaggerated or unusual perspective (usually close, but sometimes far)
  • Wide angle of view
  • Thin DOF
  • Defocus blur
  • High contrast

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    High saturation
  • Aliasing artifacts (usually described as "sharpness")
  • Lighting (e.g. chiaroscuro)
  • Good composition



For example, many folks feel that large format photography has a special 3-D look to it, which usually just comes out to wider angle of view (not unique to large format), thinner DOF (also not unique) and high contrast.