Oh yeah, how if a player wanted to play a dragonborn, it wasn't legitimate for the GM to say "No, I don't like them". Please let us not go there again.
Actually, I've been thinking about it a lot, and... yeah, I'm going there again.
Let me analogize to a judge.
A judge, when acting as a judge, has a LOT of power. She can order you locked in jail, she can decide who does or does not own property, she can even terminate your legal rights to see your own children or order you executed. We give judges this power for a reason, and choose them and position them in ways to encourage them to use this power as wisely and as ethically as possible. To put things roughly, a judge has incredible power when acting in the interests of the lw and of justice.
A judge, when not acting as a judge, doesn't have any particular power. A judge on her day off, say, at her kid's Little League game, cannot order you imprisoned. A judge, even while seated on her bench at the height of her power, cannot properly make dictates that the law does not authorize, and will be reversed or censured if she tries. In those contexts, the judge cannot do these things because she's not acting as a judge. She's acting as just another person in the room, and she has only the rights and privileges of any other person in the room.
I think that DMs are the same way.
A DM has incredible power. We give the DM that power for a reason, though, and that power exists in the context of that reason. The DM has all kinds of power because we expect the DM to have a larger vision of the campaign and the game than the players. We give the DM this power because we expect the DM to use that broader vision to make wiser long term decisions than a player might make from the player's limited perspective.
But a DM who isn't acting as a DM, like our judge on her day off, is just another person in the room. A DM not acting as a DM has only the rights and privileges of just another person in the room. Its why being the Dungeon Master doesn't entitle you to demand that your friends wash your car or clean your kitchen. That doesn't have to do with your role as a DM.
The trickier questions are things that have to do with the
game, but in my opinion, have nothing to do with
your role as a Dungeon Master. That's why I'm always so interested in a DM's motives rather than his decision itself. A DM who, say, bans dragonborn, because he has an established 3e game in an established setting and he does want to switch to 4e but doesn't want to change the setting, is probably acting as a DM. He is probably concerning himself continuity and setting coherence, and is probably making the decision that he reasonably believes to be in the long term interests of the game.
But what about a DM who bans dragonborn because he just hates them? I don't think that DM is acting as a DM. He's not using his higher vantage point to make a decision that's best for the group. He's like the judge who tries to lock you in jail because your kid beat her kid in a baseball game. He's using his power and the trust reposed in him to make a decision that's personal rather than related to the role which granted him power.
Now of course the immediate response to this is, "But if a DM hates dragonborn, why should he have to play in a game with dragonborn?" Which is fair.
But here's the thing! That's not a DM concern! That's a "guy in the room" concern. Take out the word DM. Add in Player. "If a Player hates dragonborn, why should he have to play in a game with dragonborn?" Obviously he shouldn't.
The key here is that these sorts of decisions, made not as DM but rather as just some guy in a D&D game, have less
legitimacy than decisions made as a DM. They're being made from a position that is on par with everyone else in the room. And as such, compromise or adjudication of issues that affect the DM not as a DM but rather as a player in the game need to be made from the perspective of a group of equals. Not from the perspective of one person dictating his will to his subordinates.
The only "power" a DM has to resolve "guy in the room" issues unrelated to his role as a DM is the power of the superior ultimatum- its generally harder to get a new DM than to get a new player, so the DM can make better threats to take his stuff and go home. But lets be clear, in the "ban X because I hate it" context, that's all that's happening- the DM isn't trying to bring an unruly player into line, two friends are sitting in a basement arguing and threatening to take their toys and go home if the other won't play the game the way they demand. One might own more of the toys, and therefore have superior leverage, but there's nothing going on other than a power play between moral equals.