I'm in a Lovecraftian state of mind this week, so let's just go with it. Let me ramble on about the popularity of the Call of Cthulhu RPG.
The game itself is over twenty-five years old, written by Sandy Peterson (who would later do design work on Doom, Quake, and Age of Empires), and based on the weird fiction of HP Lovecraft and other horror writers of the 1920s. It is a role-playing game, so that means you and your buddies get together and fight evil stuff, much like in D&D.
But, whereas D&D slants things towards the heroic, CoC takes things towards the horrific, producing a very different game experience. Here the monsters are very deadly and the player characters surprisingly fragile. The gods themselves are not friendly providers of clerical spells but active opponents, powerful beyond ken and uncaring about humanity (if you are lucky). Both games have hit points, but CoC also taxes the Sanity (SAN) of the player character. See a dead body? Lose SAN. See a monster? Lose SAN. Check out the Miskatonic Library? Lose SAN.
Sanity does not recharge like hit points, so the more you learn about the true nature of the world, the less stable you are. This results in various phobias and manias in the game, and ends up with your character hopelessly insane.
Not the D&D curve of increasing power and challenge at all. Yet, this is a very popular game among people who design games. I know a lot of people who design in the "heroic" fantasy of traditional and computer RPGs, that when given a chance to play, hunker down with the eldritch horrors and mind-numbing terrors of CoC.
Part of what's intriguing about the question is that the game system is pretty primitive by modern design standards. The game mechanisms are primitive, and share with the original D&D design the idea of creating a new mechanic every time they need something new. Much of what you need to know to survive combat is found in the skills section. Yet we have a player in our current CoC game who is using rules from the original edition (It is up to 6 editions now, for those bemoaning D&D's continual revisions), and except for some power creep (negligable when fighting Nameless Spawn), he's more than holding his own.
I'd put forward that the simplicity of the rules and short-term life-span of the player characters are reasons WHY Call of Cthulhu is popular among game designers. The rules are ornate but most of the time are easy to grasp, and the disposability of the characters steer the players to concentrating more on story than on personal power and possessions.
But I think one of the real attractions to the game (and it has taken me long enough to get here) is that the playing style of the game dovetails so neatly into the Weird Tales of Lovecraft and others. The name of the game is also that of a short story, where the narrator (much like a player) doesn't really DO anything but check out a list of reports from his dead uncle, who in turn didn't do much except check out OTHER peoples reports on the Cthulhu mythos, leaving the original narrator with a sense of dread and horror at the universe. Sort of like the game.
And then there are the handouts. The original CoC story is filled with things pulled from the narrator's uncle's old box of stuff, a mechanism that is repeated in the CoC game as news reports, old books, and testimony is physically passed between GM and player. The pacing of the story is matched in the game, without any need of external game mechanics to enforce it. The only comparable thing in D&D (guys meet in a bar and decide to go into the underground to fight monsters) is in Fritz Leiber's Fafhrd and Grey Mouser. But the passage of deadly (SAN-losing) information from GM to player is sort of dark communion within the game.
But all in all, I'm not sure why a) CoC is popular amoung this particular group (game designers) and b) why its not as popular beyond those borders. I don't know if it says something about the game, or about our community of gamers.
Scratch a designer, find a cultist.
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