I am trying to make an all-encompassing guide that I personally follow when I run traditional games, such as AS&SH, Call of Cthulhu, Runequest, Warhammer Fantasy, Ars Magica, Numenera, Rifts, Exalted, Wandering Heroes of Ogre Gate, Zweihänder, GURPS, Traveller, Dragon Age, or World of Darkness - any game that has distinct player and referee roles, and their mechanics are concerned more about success and failure than narrative rights. Here is what I have so far.
The setting is separate from the characters and exists independently. Locations aren't designed and power levels aren't necessarily considered with the player characters in mind. If something sounds dangerous, it probably is. In the lich's mansion you will likely run into the lich, whether you are level 9 or level 1. Similarly, numbers aren't tweaked based on the number of player characters.
There is no plot in the sense that no encounters are planned to take place in a given order (or at all, sometimes). The events of a session will naturally form a narrative, in the sense that each day or lifetime of a person forms a narrative. Some of it makes sense immediately, some of it never will. Some of it would make a good movie, while some will be a chaotic mess. Importance is not inherent but recognised either retroactively or spontaneously.
Locations, NPCs, monsters, and items are all treated as stolen cars. I don't invest much into them emotionally, and the players are free to interact with them as they please. The consequences follow the logic of the game world (which is a combination of realism, genre appropriateness, and gaming conventions).
Dice are rolled in the open, and once a roll is declared, there is no going back. If you ask, I will tell you what happens on a failure, and you can decide not to go forward, but once the die is cast, both parties have implicitly accepted all the possible consequences. Impossible or trivial things aren't decided by dice rolling, though. They fail or succeed automatically.