Its ALL about the texture and the lighting I think. I've been searching for that holy grail of rendering for ages and no matter how good the texture is, with the wrong lighting it will look fake and vice versa.  The best I could ever manage was with this image (above) but note I had to make the image sepia and add a little blur to achieve it (postwork) . Cararra's HDRI does a pretty good job of skin as well (see some of the images in the gallery at the minute by others).