In an attempt to avoid playing Cthulhu like Dungeons & Dragons, I sat down and tried to determine what was the essential difference between the two games. My conclusion was that D&D is an attempt to build self-identity and Cthulhu is an attempt to destroy it.
The particular line to focus on comes from the Call of Cthulhu Roleplaying Game by Monte Cook and John Tynes on page 203. "Once you truly understand that humanity is a dead end, the only way out is to stop being human".
In D&D, adventurers take on more and more difficult tasks and in doing so increase their own powers, skills, etc. There is an affirmation in the D&D game that the adventurers can make a difference, can change the world, can destroy evil if they choose to do so. Cthulhu has similarities in that investigators take on more difficult challenges and can gain more knowledge and/or powers. The difference between the games lies in that investigators are suppose to become more aware of their own insignificance to the broader scope. Investigators, unlike adventurers, do not grow proportionally to the broader scope of things happening around them.
Yes, the adventurer and the investigator might have to defeat their equivalent 20th level monsters, but the significance of their particular monsters are entirely different. For the adventurer, the 20th level monster is the heart of the adventurer's difficulties. For the investigator, the 20th level monster is less significant to them than was the 1st level monster when they knew nothing about the Mythos.
A potential problem with Cthulhu is finding an appropriate means of relaying that insignificance to the players. When you set equivalent level D&D and Cthulhu adventures beside each other, there is a strong chance they will look essentially the same: level-equivalent monsters, mystery, betrayals, spells, skills, NPC interactions, etc. The similarity is even more so if the D&D adventure has a theme of horror and the aberrant.
The conclusion I made is that to effectively relay such insignificance, the Cthulhu adventure has to be designed to influence the player's mind-set over the story of his or her investigator. One benefit, of course, is that designing adventures that way automatically puts the investigator-characters at a lower level of significance than the adventurer-characters of D&D. I'm not going to mention any drawbacks.
My first attempt at applying this game theory is to systematically destroy everything about the investigator, starting at the fringes and working inward, starting from 1st level to 20th level. 1. Expectations 2. Physics/Mathematics 3. Fellow human beings 4. Natural Sciences 5. Friends 6. History of Civilization 7. Family members 8. Government 9. Loved Ones 10. Courts/Police 11. Senses 12. Religion 13. Logic 14. Family (as institution) 15. Emotions 16. History of Family 17. Fatih/Beliefs 18. History of Investigator 19. Willpower 20. Profession
Each level has its own theme of destruction starting with destroying expectations until a final culmination of the whole concept of being an investigator is destroyed. Further, as each concept is destroyed it never comes back in later adventures and therefore cannot be relied upon or utilized [implementation entirely dependent on the talent of the GM], forcing the player to adjust his or her mind-set about how to accomplish the task at hand. By 20th level, assuming investigators live that long and players can adapt to such a degree, there is nothing left of what a player would normally utilize in directing his or her character through an adventure.
Presuming the theory is correct and the implementation works. What should result is that players who have progressed through multiple levels with their investigators handle the new adventure differently from a player who is just joining the group. The new player, uncorrupted by the earlier adventures, ought to be puzzled by the more experienced player's unwillingness to do certain things, even though there has been every indication that is what ought to be done. The experienced players will try to explain the situation by saying such things as "you don't know our GM well enough". The truth of the matter, however, is that an entire group of new players could probably finish the adventure doing everything the wrong way and it's not so much the GM being different as the experienced player's mind-set has changed when playing Cthulhu. I wouldn't be surprised if there was a level limit associated with each player that was well below 20th level.
Once you truly understand that investigating is a dead end, the only way out is to stop investigating.
It's a meta-game approach because when you lay the Cthulhu adventure next to the D&D adventure the similarities still exist. It's only when you look between the lines of the Cthulhu adventure, like the examination period of a Cthulhu artifact, that the forbidden knowledge is revealed.