Upload & Sell: Off
i think if ten people answered this you would get 9 disagreements, FWIW I think its a multiplicity of image attributes. Vital contributors are contrast, acuity, and resolution; the base camera requirements
Contrast is important, in my experience this is best from quality glass. Of course is also contributed too by the sensor and imaging engine. You can even recover some via post processing, so that has a role too.
We have spoken elsewhere about the sensor/lens based acuity of an image. We can fake this a little with UM, but like all things it is better to have this attribute in camera; sharp refined edges, hopefully without anything in the way of black/white edging
Resolution too provides the basis for acuity to engage the substance of the image, and chip density has improved markedly over the years. And this on its own it can exert a lot of travel for the strength of detail of an image. But this is mostly a reserve for the previous attributes, and is not solely able to provide 3D punch.
That said, you can fake these attributes at varying levels of success with Post Processing. Most photographer's would have a process that appeals to them to lift the flatness of an image. But experience shows that done without skill, or pushed beyond a given images limits reverses the process to a failing mush as we have seen.