Requiem ex Inferni
A DM's job is fourfold - storyteller, judge, opponent and entertainer all in one. He provides a story which the players, through the actions of their characters, determine the ultimate outcome of; he acts as the mediator for conflicts and has final say on how to interpret the rules; he provides challenges for the players to overcome and enemies to defeat; and he ensures that everyone at the table, including himself, is enjoying the game (a great DM is one who can put the players through the meat grinder and have them wanting to come back for more because of how much the enjoyed it the first time).
The key is balance; you have to be able to juggle those four roles simultaneously. Occasionally doing one role over the others may be necessary, but ultimately balance is the key.
This really depends on the game at hand and what the players (and the GM is a player) want out of the game.
Story teller: There are so many ways to make story happen in a game, and not all of them require prep or even a lot of GM juggling.
In
Fate, prep is centered around seeing how the PCs tick and creating a scenario with stakes that hooks into that. In
Tokyo Brain Pop the progression of events is almost entirely out of the GM's hands -- the players drive just about everything and the GM's job is to keep everybody on task and set it up so that they can drive it.
Judge: Not necessarily. If everybody is playing in good faith then interpreting the rules ought to be a group effort.
Opponent: There are plenty of games where the GM is an opponent.
D&D is one of them. But more often than not, the GM is instead a facilitator for the other players.
Like, in the
Smallville RPG the GM doesn't make antagonists in order to challenge the PCs, they make antagonists to drive wedges between the PCs. The PCs, responding to the disruption of the status quo, fight and bicker and hurt each other, because that's how you grow your character in
Smallville is by getting hurt.
In
Golden Sky Stories the GM is not even supposed to think about challenge. They're there to create a dilemma that will, through the players fishing for XP and using magic powers, create cute, emotionally resonant, and colorful scenes.
Entertainer: Entertaining the other players is not solely the job of the GM. Expecting to be entertained at the table without putting anything in yourself is a selfish way to play and, often, a drag on the other players. Everybody at the table should work toward the entertainment of the other players, whether that's through working together to overcome challenges, pitching in to develop a world that everybody can immerse in, or creating a powerful story together.
Everybody should be engaging in everybody's fun, not just the GM. A great GM is one who can help bring everybody's ideas of fun together in a way that everybody can enjoy.
The GM only has 3 absolutely mandatory jobs and the first two jobs are also the job of every other player at the table.
Honestly facilitate communication and help to meet the other players' needs.
Make the PCs' lives not boring. Make stuff happen and engage in the stuff that each player came to do.
Portray the world faithfully.
And there's different stuff that goes into these three jobs which the only the GM needs to worry about. A couple of these are in my post up-thread.
I suppose my point is, the GM is just another player and their responsibilities are variable depending on the needs of the game and the players. Communication is key, because if everybody's communicating their needs and listening in good faith then everybody can help cover each others' weaknesses, including the GM's.