jlv61560

Okay, thank you.  Perhaps we should change the topic to "Streams of Consciousness."  What say you, moderator? 

Well, to return to the original question, and speaking as a considerable fan of Call of Cthulhu (the game) and anything written by HPL, I think for me the difference is best described by the word "atmosphere." 
 
Clearly Indiana Jones is a globe-trotting adventurer, in the classic mold of people like Doc Savage (if not so "super-powery"), while the average protagonist of a Cthulhu Mythos story is more like Marcus Brody.  Except that Marcus Brody doesn't know anyone even remotely like Indy, and instead has to deal with the situation on his own.  And except that the situation doesn't involve murdering Nazis stealing an ancient artifact as the primary opponents, but monsters so beyond the average mortal's ability to deal with them that the most frequent recourse for the protagonist is either death (often by suicide) or a rapid descent into madness.  Even when purely mortal cultists are the primary enemy (as in in the eponymous Call of Cthulhu itself, they are actually not considered the worst part of the story; but merely lesser manifestations of the incredible evil and corruption of the primary foe (that being whichever Great Old One the protagonist is being forced to confront).
 
Nor is any of what's going on really "magic" -- even though it may be described that way by mere humans; but instead is a manifestation of Clarke's Law though more focused on mathematics and physics than merely on technology.  Or, to rephrase Clarke's Law:  "Any sufficiently advanced mathematical, geometrical or physical sciences skill is indistinguishable from magic."  Given that HPL was an atheist, his "magical systems" (at least according to his letters) were intended to merely be mysterious manifestations of outre math and scientific understandings -- indeed in Dreams in the Witchhouse, he clearly makes that connection as Walter Gilman both slowly goes mad and increasingly interacts with Keziah and Brown Jenkin as his understanding of "non-Euclidean" geometry grows.
 
So while it would clearly be possible to interpret Indiana Jones adventures in terms of the Cthulhu Mythos, they would cease being the free-wheeling pulp adventures they are, and would instead become dark stories of cosmic horror in which Indy and his friends would be consumed by horrific deaths at the hands of frequently nearly indescribably monsters and beings and the survivors would steadily descend into abject madness more as a defense mechanism than anything else.  Which brings us back to the "atmosphere" argument I launched in the beginning.  In Cthulhu Mythos stories and games, the world is a terrible place, concealing inconceivable evil and terror which can only be "defeated" by nearly impossible efforts, usually costing the protagonist either his life or sanity, and which really only constitutes a minor setback for the entities being opposed.  In Indiana Jones, a single (talented, but basically normal) human opposes and overcomes more common "evils," albeit with many "cliffhanging" opportunities along the way, and with the help of some loyal, but probably not terribly impressive friends.  Indy suffers no real negative consequences of his confrontations with his enemy; he's not driven insane, nor is he killed (though he may take a beating along the way), nor is he left with deeper questions as to the meaning of it all. ("What's that?"  "The Ark of the Covenant."  "The Ark of the Covenant?  Are you sure?"  "Pretty sure." -- and thus a major event in Indy's life is dismissed in one flip exchange.  Whereas in a more Mythos-oriented tale, merely seeing the image might be enough of a shock to send him fleeing from the tunnel he's in towards the more sane light of day.)
 
In game terms, it's why Call of Cthulhu has a "Sanity" mechanism (which will eventually and unavoidably remove any character that physically survives the game through guaranteed madness), and D6, Pulp Hero, and so on doesn't really (though you could certainly add one if you liked).  I note in passing that in Chaosium's house organ for Basic Roleplaying, Uncounted Worlds, they have replacement rules for "Stress" instead of "Sanity" which removes the inevitable descent into madness from the game and replaces it with a more "shock" -like mechanism suitable for more pulpy roleplaying -- and which, unless over-damaged through lack of time to relax and heal, can be completely recovered from with no lasting ill effects.  Whether "Stress" is any more a description of reality than "Sanity" is, I'll leave to the sufferers from PTSD to determine.
 
Anyway, that's my $0.40 worth (that much, because I probably went on about 20 times as long as anyone wanted me to).  ;-)