So, should the average PPL holder understand that class D VFR needs a flight plan? Let's look it up in a typical PPL level text book. How about the AFE course written by Jeremy Pratt? Lots of people learn from that.
Air Law p67 (of the second edition, 2000):
"a flight plan must be filed in the following instances:
... for a flight in class A airspace
... for an IFR flight in controlled airspace (Classes A to E)
...for VFR flight in Classes B, C and D airspace. This may be an abbreviated flight plan passed over the radio."
That's pretty clear and straightforward it seems to me. And clearly the average PPL should know.
And PA28181, you have filed a FPL for flying in Class D - you filed an abbreviated flight plan by talking to ATC about your intentions on the radio and requesting transit.