I wrote this in a response to a post on the excellent Tankards & Broadswords blog. Badelaire began with a wholly valid point:
I think the Number One Problem is that, just as you cannot easily translate a book into a movie with any degree of success, or a movie into a book, it is much harder than it might seem to successfully turn a book into an RPG campaign setting
By the end, he'd reached this conclusion:
The more I think about it, the WORST thing you can do to your favorite fictional setting is to turn it into an RPG campaign setting. In the harsh light of day (i.e., after "statting it up"), the love of your life...well...she probably ain't so pretty no more.
Which, I would (and did) say, is completely missing a more important point. If I give Cthulhu hit points, X attacks per round, Armor Class, etc. then the problem isn't that I chose to stat Cthulhu out; it's that I used the wrong *kind* of stats. You don't pound in nails using a screwdriver, and you don't quantify a Great Old One using mechanics that were designed to represent entities players could kill with axes and fireballs.
Meanwhile, if I give Cthulhu stats such as "Health: Cannot be harmed except by GM fiat, in which case he reforms in 1 round if 'injured' or 5 rounds if 'destroyed'" and "Attacks: Can devour the nearest 1d4 PCs as a standard action", then I've managed to come up with a set of mechanics whose results will be true to the source material.
Translating fiction into game mechanics is how roleplaying games are made in the first place. It produces terrible results because you did a crappy job on the translation. Trying to turn your favorite piece of fiction into an rpg setting is the BEST thing you can do- specifically, the best thing you can do for your abilities as a GM and RPG designer. Because you'll be better equipped to see all those horrible ways in which your initial attempts fall short, and have some extra motivation to try, try again.
P.S: Naturally, telling the players about something's statistics can still be bad idea. Deciding what kind of metagame information to pass on is a separate matter- and a very interesting one. Ever play in a game where only the GM knew everyone's current hp?