These are the phenomenons I see without thinking too much about the image, all doable with mental ray:
- SSS sub surface scattering.
- Skylight or a large area light behind the camera, no hard shadows. The light has a strong falloff, objects in the back are dark.
- Perpendicular/parallel falloff map, faces parallel to the camera have a white color, and the more perpendicular don’t.
- A lot of Ambient Occlusion, either naturally with the final gathering or artificial.
- The top left looks it has a bit of transparency but I might be mistaken, the image isn’t very clear.
- DOF, while not very shallow, its noticeable.
- I’d use an object with a high poly count for the irregularities to be affected by the falloff map of the shader. I don’t know if you could achieve this with normal maps, so I’d stick with a high poly count.