...'cause goodness knows we have them. Maybe that's one of the symptoms.
But seriously, the very fact that your question is a difficult one ( i.e. what are our individual major symptoms) tells us that we are dealing with an unusually complex mess. My vote would be for cutting through the confusion of the symptoms in search of the cause. Unfortunately for us, treatment of symptoms generates far greater profits than cures and altruism is not a corporate quality.
So it's left to freelancers like us to make the attempt to sort it out. And I really do believe it is possible. I have immersed myself entirely too deeply in the subject for the last four years and, while my head spins at times, I am convinced that the data is there but it is hard to reach for three reasons- 1) the economics just mentioned is one; 2) the sheer volume generated over the last twenty years has greatly slowed the integration of new information into the whole; and 3) PD falls into the cracks between neurology, endocrinology, immunology, and gastroenterology. Over just the last five years, researchers like Carvey at Rush, Bin Liu and J.S. Hong at the NIH, and a dozen others have published findings that make convincing arguments that of the four, neurology comes into play last as the other three factors damage the nervous system.
But how in the heck are you going to convince a room full of neuros of that? They don't have time to keep up with the literature of their own profession. Data floods in so fast. They can't be expected to be up on the effects of the endocrine system - even though their patients are so hugely vulnerable to stress. Or the immune system - even though neuroinflammation is almost universal in PD. Or the GI system - even though the roles it plays are too numerous to list here. Yet, as Dr. William Langston pointed out in the PDF newsletter, PD is only going to be solved by multidisciplinary teamwork.
In short, even though research must go on, it must be merged into the existing knowledge-base if it is to have value. That's why I am advocating an online think tank in the "private" forum. Cheap and accessible, it could draw researchers from all these disciplines and from all over the world. It could run indefinitely and act as a constantly updated snapshot of what is known. Most importantly of all, the bits of treasure that might emerge from this data mine could be of immediate benefit to those of us fighting right now.