Besides the custom tweaks and the posing, another important aspect of making a good looking render is the way of how you light the model and the scene. I've seen a lot of devs who only use on-axis frontal lighting, resulting in flat looking faces with ugly highlights and pinpoint catchlights smack in the middle of the eyes. Even the inside of the models mouth is lit. This is the equivalent of talking a picture with a pop-up flash. You can get away with it if your name is Terry Richardson, but most of the time this does not result in a flattering picture.
Other devs go even further and render scenes which seem to have been created inside a light box. No direction of the light, no light falloff on the background, no contrast, no depth.
Naughtyroad and a few others on the other hand, manage to light their scenes in a way that is both realistic and enhances the mood. It is also flattering for the models, making them look more attractive. As an amateur photographer, I tend to notice such things.