"The advantages which have been derived from the caution with which hypothetical statements are admitted, are in no instance more obvious than in those sciences which more particularly belong to the healing art. It therefore is necessary, that some conciliatory explanation should be offered for the present publication: in which, it is acknowledged, that mere conjecture takes the place of experiment; and, that analogy is the substitute for anatomical examination, the only sure foundation for pathological knowledge." 

"When, however, the nature of the subject,and the circumstances under which it has been here taken up, are considered, it is hoped that the offering of the following pages to the attention of the medical public, will not be severely censured. "      James Parkinson, London, 1817

While it is common to think of problems with movement when one thinks of PD symptoms, there is much more. So much more, in fact, that the term "movement disorder" quickly comes into question.

From http://www.pdindex.org/, for example, there is:

It is somewhat misleading to label these as the "primary" symptoms as though they were most important. It is often the "secondary" symptoms that most impact the patient. Depression, for example, can be far more devastating than, say, tremor. In addition, the categorization of relative status of symptoms would vary were age and gender factored in.

I understand that there is a certain amount of semantics involved here, but that is one of the problems. Define PD by the traditional symptoms above, add in the importance of l-dopa, and suddenly you have a "movement disorder" by definition - but not by observation! And it does little to justify this semantic "tidying up" to claim that those four are universal while others are not. That is not true, for one thing, and, even if it were, the label would still be misleading.

I don't intend to belabor the point. But the fact is that much emphasis has been placed upon those "primary" factors as a matter of convenience (after all, they are certainly not subtle) and has led to the "secondary" status of some very important clues:

All these (and more) are part of Parkinson's Disease and must therefore must be accounted for by any complete theory. We do not hope to reach that level here. But the closer we come the better our work.

One final point - the complex symptomology noted above illustrates the individual nature of this disorder. And the term "disease" may itself be misleading.