How strongly should the new edition stress Rule Zero (that the rules are guidelines that the GM should feel free to nix in any given situation)?
Saying Yes and Rule Zero are two of the most essential rules for any DM and player group.
So I think there should be a place in the core rulebooks - maybe where the DM's role is explained, where both is adressed.
"As the Dungeon Master, you have a unique role and responsbility. You guide the game. You describe the world to the players, and you decide how the world reacts to the actions they take.
But not only that. You also are the arbiter of the rules.
If a rule is unclear, or something is not covered by the rules itself, you may have to make a decision. It is okay - and often advisable - to ask for input by the players and consider the general interests of the players and the group, but the final decision is yours.
Sometimes, you or members in the group will even encounter rules that you don't like, and as Dungeon Master, you have the right to change the rules to better fit your and your group's preferences. "
"As a player, you can and often should offer input and advice to your Dungeon Master to help him run the best game he can. The Dungeon Master has the final say on the rules, and if he deems it better for the game, he can ignore or change rules. He is the final arbiter of any conflict."