Very good reading material as always, El Dude.
I can't help but think that JMDP is more than a second tier player, although, yes, you are right, it is only a "could have been" (that still can be, I hope), results wise.
But one (subjective) thing I think separates him from the rest is his attitude/capacity against all members of the big four (three). He always steps up on court to take it to them, and usually the result is never a given. For me (and that's the subjective part) those matches are much more "open" than (usually) the rest of the second tier x top dogs match-ups. The list of epic matches (winning or losing) or important wins of JDMP against the big three is evidence of that. From the top of my head:
USO 2009 (semis and final)
DC against Djokovic (2012?)
DC against Nadal (2012?)
Olympics against Federer and Djokovic
RG against Federer (2009?)