The theory to employ either simplicity or anatomically correct detail in artwork.
I once read an article describing the human tendancy to find the most emotional connection with things which either exactly resemble us, or which have less human likeness and are simpler in design. A happy face, for example, is seen as cute or at least as an aesthetically acceptable symbol of that emotion, much as is a sad face. And with a person with genuine sadness on his or her face, many of us feel sympathy for him or her. But that which resembles a human, but not quite exactly, is often seen as disturbing, and much of the emotional connection is lost. Androids which very closely resemble humans have been shown to be more disturbing to us than androids like those in irobot, according to this theory. When mapped on a chart, this region of disgust for that which only closely resembles us is termed "the uncanny valley." To site credit where it's due, research on this subject was performed by Karl F. MacDorman and Hiroshi Ishiguro.
This said, I see a comparison with hentai artwork. Simplicity in character design makes for a more aesthetically pleasing image. Furries which retain the animal quality in just the right ratio may be more pleasing to the eye than furries which very closely resemble humans. However, if the artist is talented enough, great detail in shading and character design is also very pleasing to the eye. It is the artwork pieces which try to look realistic yet fall just short which are the least pleasing.
I write this because I think that this theory of simplcity in artwork is what attributes the large fanbase to Sonic and Pony pics. The ponies are so cute and appealing because they are fairly easy to draw and simple in design. Sonic art is the same. My Sonic pics tend to be more popular than pics of other characters. The talented draw their high detail, realistic pieces very well, but oftentimes when others try to imitate this style it falls short and is seen as more ugly than if it had been drawn simplier. In my own experience, adding certain muscular curves to a woman's body in a pic lowers the appeal than if they had been left out altogether, however, if the curves are added just right and to the correct extent, it can also greatly improve the quality of the image.