All of these little things that make up the ride, these are his province. So this guy has to deal with that fact. The pieces of the ride, and making sure that the ride is working, is more important than the momentary desires of any one person on the ride. He has (dare I say) bigger fish to fry.
The whole ride is more important than instant gratification, but I'm not talking about instant gratification -- I'm talking about the DM serving the player's needs. And that does mean some gratification. Maybe not instant, maybe in the proper context, but gratification nonetheless. And a codified ruleset is an aid to that gratification, because it tells you in which instance and in which context that it can be used. To use the ride analogy, a good ruleset tells a DM where to turn right and where to turn left and where to go faster and where to go slower to make sure that all the people who want to go left, right, slower, and faster all get what they need. It also tells the players that while they might not be able to expect going left right when they want to, that they can expect it to go left at some point, and they can expect it to be everything they want out of a left turn.
I said that the DM exists for the fun of the players, not as a magical genie. Having a magical genie is only fun once or twice (and it is recommended for a DM to be a magical genie every once in a while), and it gets old fast. Players don't have fun with instant gratification. Players don't want to have their wishes granted right when they demand them. The want to be challenged. So the DM's tactics of delay and challenge and risk of failure can help make the game more fun. However, a DM that only grants wishes in perverted and conflicted ways isn't challenging the players -- he's punishing them for having desires.
Does the DM have more important things to do than grant everyone's wishes? No. Players *wish* for conflict, for risk of failure, for trials and tribulations. Challenging PC's grants the player's wishes.
Does the DM have to give a PC a Staff of the Archmagi at first level just because they really wanted one? No. Granting that wish would remove conflict, risk of failure, and challenge. A good player will realize this and not really bother with it. However, the ruleset tells us when a Staff of the Archmagi would *not* remove conflict, risk of failure, and challenge. And at that point, you should have a good reason to deny it, because you denial isn't helping you build the ride anymore.
Failing is not fun. Not just death, but failing at those risks that are non-lethal, too. No one likes to fail. Perhaps we should eliminate all risk from the game, and have only the illusion of risk.
Risk is fun. Players want risk. Giving them risk is giving them exactly what they want. However, there is a point at which the risk becomes too great for the player. Where the effort involved in making a character and becoming invested in the story may be too much time and effort that could go to waste. Roller Coasters are fun preciesly because of that illusion of risk. If 50% of roller coasters ended with smacking headfirst into a brick wall, they'd be a lot less fun, because the risk is too great.
If, every time that a writer failed to churn out a successful novel, it got ripped in half, that might be too much risk for a lot of people, too. If my hours of character generation, if my investment, depends upon pure luck to be fun, then it may be too much risk.
But, the DM shouldn't say "No" to playing a big strong monster?
But the risks shouldn't be very risky?
The DM should only say "no" when playing a big strong monster would hurt the ride. There are rules that exist that tell you at what point being a big strong monster won't break the ride. And as long as the DM can have fun on the ride, too, then there is no good reason to NOT use those rules. If the DM wouldn't have fun, obviously that's a point of conflict. Normally, compromise can be reached (fer instance, Savage Species' concept of racial levels, or finding a way to make the character strong and scary and legendary without resorting to ECL). If it can't, it can't, and the player and DM can either agree to disagree and suck it up, or go their seperate ways. But an effort should be made to compromise on both sides, not just the DM's, and not just the player's.
The risks should only be worth it. In a game with a high body count, spending an evening generating a character isn't worth it for a lot of people. In a game where your character can theoretically last an unlimited amount of time, generating a character will always be worth it. Most seek a balance somewhere in between, D&D errs on the side of safety but permits the other side as well. The ride shuoldn't frustrate you at every turn -- turning left like you want only to juke right, or turning left like you want and slamming you into a brick wall. Spending time on the ride is an investment, and that investment has to be worth it. With a selfish DM, it often is not.
PLAYERS COMPETE AGAINST OTHER PLAYERS.
?
How is this? Players, IMXP, compete against the challenges set before them to reach the goals they want to achieve for their characters, and they do so together, not at odds with each other. Players help other players. The player of the wizard helps explore the cleric's crisis of faith, and the player of the cleric helps the wizard raid the dragon's hoard for his magical whatsis.
(And yes, I know that you said the DM can say "No". Yet, here I go, splitting hairs again. When can the DM say "No"? When he feels it's appropriate? After taking a democratic vote? When the players tell him it's okay? Or does it not really matter because the potential failures are so insignificant that it makes no difference what the PCs are, or what anyone chooses to do anyway?)
The DM can san NO whenever saying YES would ruin the ride. In other words, whenever he's
justified. Players are entirely entitled to a justification for the DM's actions, just like a DM is entitled to a justification for the player's actions. Why do you want to play the big monster? Why don't you have elves in the world? These are fair questions that any DM should be prepared to answer, because there is a set of rules about how to let that drunk guy on the ride and have him have fun too. Got a problem with drunks? Fair enough, he can look for a ride that won't discriminate against him. Know your ride has too many ups and downs for a drunk? Sure, let him know that if he doesn't mind sobering up, he can come back.
GM: "Okay, go ahead and create your characters. All characters are 2nd level so you can pick any LA+1 race on your handout..."
Player 1: "I want to play a demilich."
GM: ( ) "Uh, that's not LA+1. All the characters should begin with roughly the same level of ability..."
Player 1: "I want to play a demilich."
GM: (*sound of fingers drumming on table*) Uh, no, I'm sorry, you can't play a demilich..."
Player 1: "WHY MUST YOU CRUSH MY DESIRE TO PLAY A BIG STRONG MONSTER?!?"
Player 2: "I want to play a tarrasque."
GM: (sound of head hitting table followed by sounds of paper tearing for several minutes*)
This is an example of how a selfish player won't accept that he could hurt the fun of others.
Here's an exmaple of how a selfish DM won't accept that he could hurt the fun of others:
GM: "Okay, go ahead and create you characters. All characters are 2nd level, so you can pick any LA +1 race on the handout."
Player 1: "Hey, there's only humans on the handout?"
GM (fingers drumming): "King Arthur didn't need elves, why do I?"
Player 1: "But I was thinking of playing a fey-like character with ties to lake maidens and stuff..."
Player 2: "And I wanted to be a wizard, but this handout says they're forbidden?"
GM: "Magic in D&D is too powerful, so I took it out. Deal."
This presents such an extreme, almost Manichean viewpoint that it's effectively useless as a point of discussion, but I'll give it a try.
With respect to character races, is a GM who restricts characters to playing races in the PHB a selfish GM? What about any LA+0 race from the MM? Any LA+0 race from any WotC monster book? What about LA+2? Is there any continuum on this subject, or is it as black-and-white as, "Any GM who limits player choices on character race selection is a selfish GM?"
I'm really trying to understand exactly what the issue is here. Help me out.
A selfish DM is the one who puts his own pleasures in the game first, ahead of the player's, rather than equal to them. Like a rules-lawyer is a selfish player who gets pleasure correcting others and won't stop, a selfish DM can be that DM who gives a player a familiar only so that they can kill it, torture it, and maim it later. Or that DM who has epic-powered NPC's swoop in and rescue the party. Or that DM who drops hints about going into the Forbidden Forest, when you get there, proceeds to TPK the group because "THE FOREST IS FORBIDDEN!" It's the DM that
needlessly limits player choice. The DM who doesn't consider the ramifications of his changes. The one who fudges for monsters and important events, but fudges against players. The one who removes spells simply because they challenge him. The one who hands out loads of treasure to his girlfriend. The one who insists that he knows the game better than the designers, and who makes arbitrary changes to "use the d12 more often." The one who railroads relentlessly. The one who won't let you act until his villain is finished with his speech. The one who demands two written pages of character history only to give you the prospect of your long-lost sister coming back only to kill her out of some delightful malice.
A selfish DM isn't interested in making a ride for other people, just in making a ride for himself.
First, a character death can be fun. A heroic sacrifice can be fun. A death that hinges on a climactic encounter can be fun. Taking the risk of death out of the game makes dangerous encounters dull and uninteresting.
Second, I enjoy RPGs that emulate a genre of cinema or literature that I enjoy: swords-and-sorcery, modern action-adventure, space opera. The risk of permanent death is present in each of these genres, and a game that did not offer that risk would be repetitious and bland - "Oh, it doesn't matter if we lose the fight with the dragon, we'll just be resurrected later."
Third and last, I feel cheated if my choices as a player had no consequences, with death the ultimate consequence. That last thing I want when I play is to be cheated by the GM. I think only a selfish GM would take that card out of my hand.
Right on all points. But a selfish DM who had the same kind of outlook you have taken to a self-centered extreme may be more concerned with "making it feel like a space opera" (for instance) than "making it fun." So death would be an ever-present threat, and PC's would be killed quite often, and then when the players didn't put as much effort into their third or fourth characters, the DM would get mad and the post a thread about how D&D 3e is ruining creativity on ENWorld, because none of his players put any effort into their characters.
For a selfish DM, their own fun of game feel trumps a player's fun of getting invested in their character. With a good DM, a balance can be found ("character death is rare in PC's because of action points, but the threat is constant" for instance. Or "there isn't a lot of combat anyway, because our group enjoys the storytelling side more" for another instance). But the rules do force a DM to at least explain themselves and consider their changes. "Because I'm the DM!" is not really a good enough answer. "Because I have fun killing characters" is a good answer, albeit not a group whose style I'd like.

"Because I want to evoke a space opera style" is a good answer, and one that I might consider joining as a player.
For that guy who wants to play a demilich? "Sorry, they're too powerful. But maybe you'd like being a necromancer...if we get high enough level, you may have the opportunity to become a demilich." Or maybe even "Well, it doesn't need to be second level....does everybody think starting at level 22 is a bad idea?" This changes the ride, but keeps fun for everyone intact. Helping the players to have fun is the DM's job. This job includes finding out what they REALLY want, which usually isn't just power, because players don't have fun when they're all powerful unless they're selfish players.
Most people who want to be a demilich or a terrasque or a dragon want to do it for specific reasons -- the breath weapon, the idea of being a floating skull character, intimidating villagers...a DM will build a ride to that does all these things, as much as he can have fun DMing.