Your question still intrigues me, prof. Mingarelli. One person who might still be able to answer it outright is Freeman Dyson.

However, in the 1986 Dirac Memorial Lectures (held in Cambridge, England), RPF says the following (I attempt to transcribe from the lecture, please bear with me):

"When I was a young man, Dirac was my hero. He made a new breakthrough, a new method of doing physics, .. was to guess an equation. Previous to that Maxwell got his equations, but only in an enormous mass of gearwheels .. and so forth .. to understand what he was doing. But Dirac had the courage to simply guess at the form of the equation and try to interpret it afterwards."

RPF then goes on to explain PAMD's process in coming to his renowned results.

Beyond that, this lecture might be of particular interest to your question given the length at which Dirac's work is mentioned and the importance given to the relation between RPF and PAMD.

Historically, due to Sakurai's quantum mechanics , the path integral ideas of Feynmann was inspired by a sentence of a book related to PAMD, this may be assumed as the origin of mentioned  "awe" .

i remember to have read in the books by Jagdish Mehra on the historical aspects of QM what Manouchehr Amiri said in the above answer. PAMD almost gave the expression for the probability amplitude for a particle to go from one space time point to another in terms of the action integral. it seems he was thinking on the role of Lagrangian in QM. RPM developed the idea completely thereby giving rise to the now well known path integral formulation.  

An alternative approach to answering your question could focus more on 'simple' human nature. I remember reading an anecdote regarding a meeting between PAMD and a young RPF, related by prof. Anthony Zee in his Quantum Field Theory in a Nutshell. I read this work to feel less lost in the company of certain physicists.

The anecdote relates that in this meeting PAMD 'put RPF in his place' (my interpretation), presumably saying "I have an equation. Do you have one too?", which might explain continued admiration to a degree, if not necessarily the origins thereof. Speaking from personal experience, having personally met and conversed with someone of equal or greater intellect and experience in one's own field of study can inspire life-long admiration.

I do apologise if in my eagerness to answer your question I misinterpreted your intent. I hope the source of the anecdote at least provides you with a potential alternative path of inquiry.

Referenced:

Anthony Zee, Quantum Field Theory in a Nutshell (Princeton University Press, 2010, 2nd edition)

So far, so good and thank you all three for your replies. It seems that RPF had few "equals" so I really am thinking about Joas' "continued admiration" idea being based on something more tangible ...but where did it all begin? Was it PAMD's positron idea? the path integral formulation idea? It may be hard to pinpoint the first moments but it's worth a try.

Your question still intrigues me, prof. Mingarelli. One person who might still be able to answer it outright is Freeman Dyson.

However, in the 1986 Dirac Memorial Lectures (held in Cambridge, England), RPF says the following (I attempt to transcribe from the lecture, please bear with me):

"When I was a young man, Dirac was my hero. He made a new breakthrough, a new method of doing physics, .. was to guess an equation. Previous to that Maxwell got his equations, but only in an enormous mass of gearwheels .. and so forth .. to understand what he was doing. But Dirac had the courage to simply guess at the form of the equation and try to interpret it afterwards."

RPF then goes on to explain PAMD's process in coming to his renowned results.

Beyond that, this lecture might be of particular interest to your question given the length at which Dirac's work is mentioned and the importance given to the relation between RPF and PAMD.

Thank you! I spoke with Dyson a few years ago but didn't think about asking him then as I only thought about this recently. Instead we talked about Hardy, Littlewood and other such giants.

Dirac was the greatest physicist of first half of 20th century even greater than einstein so naturally Feynman had high respect for him

Have you seen Gino Segre's book Faust in Copenhagen? It shed quite a bit of light on the lives of the founders (mostly men but also Lise Meitner) of quantum physics, and may offer some insights about Dirac especially. In any event it's an entertaining read.

Thank you all for your comments and contributions, they are much appreciated.

I've just watched Feynman giving the Dirac Memorial lecture in 1986 and this is probably the closest we'll ever get to answering my question. Perhaps the fact that Dirac used pure mathematics to answer a question about physics was what made Feynman regarding him as truly exceptional. It's worthwhile to note that RPF always based his ideas on unparallelled physical intuition, with the math coming afterwards, but it seemed to be the opposite with Dirac at least in the case of antiparticles.

Did Dirac really guess though ? Freeman Dyson in Advanced Quantum Mechanics says his "guess" was heavily constrained at least by invariance requirements.

Well a guess here is not an ordinary guess, it was a piece of brilliant insight, sometimes it works!