People generally agree that the nude female form is more beautiful than the male. The arguments generally cited for this include that women have more curves and less obvious musculature, or that male bodies are more 'functional'. Whatever the reason, couple it with the fact that (particularly historically) most artists are male, and you have a large preference in photography for nude photographs of women rather than of men.
But what makes a good nude female photograph? There are three main principles.
#1: It should be interesting to look at
Nude photography is synonymous with that branch of photography known as 'fine art'. Fine art photography can be defined as 'photography created for no other purpose than because it's interesting to look at'. As such, nude female photography is about presenting the female form in a way that is interesting.
In order to make an interesting image, nude female photography tends to favor black-and-white images that are full of light, shadow, shape, and contrast. There are usually carefully composed and lit in order to achieve this.
#2: It should be anonymous
Nude photography is not 'about' the person that is in the photo - in fact, the face of the model is often not even included in the frame. Contrast this with portrait photography, where the idea is to reveal or display something about the subject's life or (more often) their personality. Nude photography is really the opposite of this, because it's about the female form in general, not this person in particular. In nude photography, the person in the photo really serves only as an example of a category, with the category being 'women's bodies'.
#3: It should be non-sexual
While any heterosexual man would enjoy looking at nude photographs of women for reasons that not entirely dispassionate, there is still a difference between nude photography and erotic photography. While the difference might be a slight one, it's all about intent. Nude photography fits that fine art definition of intending only to produce 'something interesting'. Erotic photography, on the other hand, is intended to titillate or arouse the viewer, and might not be all that interesting.
Given that what is 'non-sexual' to one person might be sexual to another, it is of course impossible for a photographer to know whether their intent to produce a nude photograph rather than an erotic one will be recognized by the audience. For this reason, at least in Western countries, certain conventions (such as leaving the genital region in dark shadow) are followed to provide an indication of the intent behind the photograph.