I'm not talking about running the game. I'm talking about assembling the game. Everything up to "we just started session 1."
There are patterns here. We can, in fact, actually talk about them and try to resolve this seemingly-irresolvable conundrum. As with previous statements, it may not work. However, I generally hold out hope that two people both attempting to discuss something rationally can work out that what they thought was pure loggerheads was actually two people having slightly different starting points. Once those are accounted for, seeming logical contradictions can be resolved, rather than retreated from.
So, to re-state my previous stuff. This is my attempt at a general model for how things happen for non-pick-up play, where DMs put lots of hard work into building a world that they'd rather not just slap-dash rewrite repeatedly:
1. DM has an idea and builds it into a campaign premise
2. DM seeks out players interested in this idea (putting out a notice and accepting applications, talking to friends, whatever)
3. If enough interested (and desirable) players are found, DM gives a more fully-articulated pitch (the aforementioned 20-page primers etc.)
4. Players develop ideas, and ask the DM questions about the pitch (possibly part of Session 0)
5. DM works through any wrinkles in the final PC concepts and approves specific players to join (also possibly part of Session 0)
6. Play properly begins with Session 1
As I said before: it very much sounds to me like those DMs here who consider it "demanding" or "disrespectful" (words actually used in this thread, mind) to do something like asking to play a dragonborn are assuming this thing happens at step 5 or 6, where everything that should have been said has already been said, and it WOULD be rude to demand something different, it WOULD be a breach of agreement and social contract to pitch a fit about not getting to play an elf. Whereas for me--and most of the people who agree with me here--it seems that we're sitting at step 2 or maybe 3, where there HASN'T yet been a full-throated articulation of what's up and it's COMPLETELY reasonable for players to have ideas that aren't necessarly 100% copacetic with the DM's "vision," where expecting a little negotiation and conversation is not rude but rather polite and REFUSING to have them is what is rude.
So. Assuming that we're not talking about this allegedly-insanely-common approach, where someone puts potentially years of work into developing a setting but then solicits completely random strangers at the FLGS to play with them, does this seem reasonable to you pro-restriction folks? Does this help explain why I have pushed so hard for these polite conversations and find it flabbergasting that people refer to it as "disrespectful" or respond to them by waving around claims of "Ultimate Authority"?
I will agree that we go through basic steps. Look for players, explain (or discuss) campaign, make PCs. I've never used the words "demanding" or "disrespectful" describing a player that I remember, it's reasonable for a player to ask questions, including whether an option is available. It's also reasonable for the DM to decide the answer.
Now, I have had recent experience with someone that I thought was disrespectful. The person joined my home game (knowing that it was going to be a long-term campaign) and then after the third session let us know that he just had gotten word that he was being transferred out of state. Oh, and that the transfer was overdue and he always knew he would only be able to play a few sessions but just never told me.
To me, that was disrespectful. Not that he was no longer attending my game, but it was disrespectful that he started a campaign intended to be more than a few sessions when he knew he wouldn't be around for it; that he took up a seat at my table when he knew that I had a waiting list. It wasn't just disrespectful to me - it was disrespectful to everyone at the table who were enthusiastic about establishing connections with the other PCs at the table. There's a social compact when joining a home campaign that you will give it your best shot and that you are there to support the campaign.
I think there's another implicit agreement when joining a game. That the DM is the person who makes the final decision on just about everything but what the PCs do* ... and even that has it's limits. I don't allow evil PCs so I'm never going to tell you that you can't do something, but I will let you know before you cross the line that the PC will become an NPC. It's not that a player can't question or discuss things but if it's more than a quick clarification do it outside of game time. If you disagree either accept the DM's answer or move on.
*This even extends to background and story for the PC. The DM has editorial control over NPCs, actions, events and locations. Whether that editorial control is used is up to the DM.