Every game setting has their own principles, though naturally GMs tend to tweak them, and I think it's a great idea to make such things explicit. Call of Cthulhu has something similar (at least in 6th ed.) which involves a sort of "What to Expect" section which is directed at players, yet still useful to Keepers.
While each game has its own principles, I thought I'd pop in a few that should be present in my Masks of Nyarlathotep campaign (which is one of the few that I'm running at present, breaks are delicious).
The Place Should Feel Real.What's the point in going to exotic locales if it's just another Generic City? I do my research, figure out transportation between key locations, and do my best to make each place feel and sound genuine. I even occasionally make a meal that was common in that era / location, which thus far, includes types of sandwiches I read about in New York menus.
The Characters Should Feel RealWhile not every character needs to be wacky or eccentric, they each should have a very real sense of being wholly and completely themselves in a way that draws attention to their traits. This ranges from Elias Jackson's coolly manipulative journalism where he quietly and gently makes offers you can't refuse; to the youthful companion Charlie Adams' cheerful eagerness; to Jean Robbins (from the Half-Moon Cult adventure) being rational and reasonable in how she deals with the PC's blackmail attempts. Each character is given as much depth as I can muster.
The Characters Should Feel IntegratedPart of the benefit of doing what will at least be a three-part prologue (London Awakening, Half-Moon Cult, The Midas Cult) is that I have plenty of opportunities to introduce characters that will become very important later on. James has already met a pathologist, a journalist and a dodgy newspaperman who will all come up later on. He will also end up revisiting a British Psychiatric Institution again as well.
Sanity Is Always In QuestionBy introducing a character from another genre / setting (Vampire: the Requiem), I have immediately given myself a platform to consider the intersection of the occult and sanity. In a world where there is definitely both magic and other realms, is it insane to think you were a vampire in another version of Earth? Yet in a world where magic shatters sanity there is a very good chance that if you've been dealing with it for awhile, you are insane. Other characters, including potential companions, will also help him confront that possibility.
Death Ends AttachmentAs a solo game, I can't really kill off the main lead without stalling the campaign and having to start over, so I have set up a situation where he doesn't die for good. He has to go through a hellish dreamscape before pulling himself together again. On the same note, I know my player isn't a massive horror fan (and abhors tragedy), so by making the side characters feel real I have to take some responsibility for his emotions when dealing with them. If I randomly slay these characters, he will feel cheated. Thus much of the horror will involve visceral threat, characters-in-jeopardy and psychological hazards, rather than a high risk of wanton death (at least for the major attachments).
Fear and Feeling Are The GoalsSince the aim of the game is to induce emotion in the player, including and especially fear, I don't really mind reducing the lethality of the game to engender that. While it may seem counter-intuitive, the fact is that some players can open themselves up to the reality of the world better when they're not trying to keep their psychological distance from everything within the game. Therefore I will aim to keep the tension at certain points (though not constantly), and whatever gets me to those points without over-stepping them is just fine. If that means slaying an NPC, then fine. If that means keeping them alive, then fine as well.
So yes, these are my principles for my Masks campaign as a GM. What are yours?