Ruling out the stones, try landscapes, and accent plants, if you were to enter any one of the trees shown on this site in a "Bonsai Show" I seriously doubt anyone would be able to say this tree is Penjing and this tree is Bonsai. Maybe it is because our awareness of style has become vastly broadened over the last couple of decades to the point that, our recognition of this being one thing and that being another, is more an arbitrary judgement than an exercise in reality.
One problem I have, and I am the first to admit that it may be more to my own ignorance and lack of concern to go looking for the evidence of what I object to; there seems to be no source of information, or "Penjing book" out there along the lines of John Naka's, or Yuji Yoshimura's books that extrapolate the artistic elements of Penjing or the rules, if any, that apply to the style or form.
I have in the past seen a few "Crappy" trees passed off as Penjing, but the same can be said of the ubiquitous American Style bonsai. If we use this guideline the term "Crappy" would be the only identifiable feature that separates the two, or three styles and I don't buy that. You have brought up a good point that needs to be addressed but is little replied on because most people don't know and those that do wont say; at least that is what I am lead to believe at this point.
If you want to identify a void that needs filling this subject is it.
