What do you consider to be the chief problem in the field of dermatopathology?

There is a very big problem in dermatopathology that also is prevalent in the field of surgical pathology.  I believe it is best stated as:  The problem of understanding the natural history of disease versus one disease "transforming" into another disease.

Getting this wrong truly "slaps" the law of causality in the face and makes it very difficult to communicate the meaning of the natural history of disease.

A dictionary definition of the natural history of disease is as follows:
. . . the natural development of something (as of an organism or disease) over a period of time (Merriam-Webster)  "natural history." Webster's Third New International Dictionary, Unabridged. Merriam-Webster, 2002.http://unabridged.merriam-webster.com (6 Apr. 2010)
The simplest way I can illustrate the meaning of natural history is for one to consider his own life; he is born and develops into a child, then to an adolescent, then an adult, then ages, then dies.  At every point he is still himself.  The changes in his structure and development are due to the natural history of the kind of organism he is.
For pathological cellular proliferations, neoplasms in this case, the analogy is similar.  The natural history of benign neoplasm is such that, if left in the host, it will not kill the host unless it impinges on a vital structure that is necessary for life (referred to historically as "malignant" by position).  The natural history of a malignant neoplasm is such that, if left in the host, it has the potential to kill or does in fact kill the host by local destruction, metastasis, wide dissemination (if a leukemia or lymphoma) or combinations of these.
The pathologist's goal is to learn about the natural history of a given neoplasm by observing usually many examples with similar morphology.  This kind of observation helps characterize the structure of the lesion, but it takes more, an understanding of its biology, to understand whether it is benign or malignant.
The best examples of this problem are illustrated in the pathology of melanocytic proliferations, which encompass melanocytic nevi (benign) versus melanomas (malignant).  There is a "school" of dermatopathologists, especially in America, which states that there is such a thing as an "in between" lesion that is neither benign nor malignant, and employs the terms "borderline melanocytic tumor", "melanocytic tumor of uncertain malignant potential",  "dysplastic nevus" and similar kinds of terms to stand for this putative intermediate.
Yet, this is false.  Any given neoplasm is either benign or malignant (or one may not know; this does not change the fact that the lesion is one or the other).  This school confuses mimicry structurally with an "in between" lesion biologically, conferring the idea that the lesion somehow doesn't "know" what it is, which is patently rediculous.  Even if a lesion has a rather indolent biology but is later found to kill by its nature, it is malignant.  Yet, benign proliferations of cells can sometimes mimic malignant proliferations of cells and vice versa.  This is why the art and science of dermatopathology are so challenging.  Understanding the nature of a lesion, the diagnosis, is a type of identification that requires years of practice.
Throughout the literature of such lesions, it is common to encounter instances were one writes of benign neoplasms "transforming" into malignant neoplasms.
The neoplasms, however, do not "transform".  They do not change from one kind of proliferation to another anymore than I can "transform" into a hound dog.  Cells within the host, however, can transform from "normal" to cells of an abnormal proliferation, and cells within an abnormal proliferation can transform via genetic mutation and a host of other mechanisms to result in a different kind cellular proliferation.  Yet, a given proliferation itself does not transform.
The problem here is that the natural history of disease has been ignored or misunderstood.  The solution is that instead of the lesion being "transformed" it has really been misidentified.  The identification of a proliferation of cells as a given kind of lesion is one of the identification of it as one of a group of similar lesions with a similar natural history.  This identification is the diagnosis.  The diagnosis is a name that stands for a given kind of cellular proliferation including its natural history.