I understand not really having a reason to trust an unknown DM. But if you don't know the DM, do you really have a reason to not trust her?
I'm all for benefit of the doubt but, trust is earned. Trust is not automatic and I've had WAY too many bad DM's for me to just take it on faith.
Let me return the question, why don't you trust the player?
Anecdotal. My anecdotal rebuttal is "not in any campaign that I have seen".
Sorry, misstatement. Monster characters are extensively covered in the rules of 3e and as such, are frequently considered on the menu at tables.
Perhaps I'm weird, but as a player who did show up to the game fully accepting the advertised parameters with my legit character, I would be really frustrated with the player asking why.
As we sit here on page 40 of a thread asking why, one sees that we tend to get a bit...lengthy in our discussions. If I show up to a game, I really want to play. I don't want to deal with the player who cannot accept the game as is.
As to discovering elements of the game, once again, perhaps I'm weird. But I really like an element of surprise. I'd like to discover the "whys" through my character exploring the setting. I don't really want to know if the main bad guys are going to be tieflings or whatever up front.
I just don't see the point of arguing about metals unless your entire character concept is somehow completely revolving around said metals. And really, if that is the case, then why go to that game?
Just my bit. I'll go back to quietly reading the thread without responding now!
See, to me, this is how this conversation should go, using the Tolkien example from above.:
Player: Mr. DM, I've got this concept that I'd really like to play in your game. I want to play an Orc.
DM: Well, that concept doesn't really fit with what I want. See, most of the campaign is going to take place in Gondor and orcs are basically killed on sight. I'm not about to rewrite 3/4 of my campaign. I'm sure you understand.
Player: Oh, gee. I really wanted to play this concept.
DM: Well, what was it about orcs that you liked?
Player: I don't really know a whole lot of Tolkien stuff. I saw the movies, sure, but, that's about it. I wanted to do the whole "fish out of water" thing and I thought orc was the way to go.
DM: How about hobbit. They don't travel, that would work pretty well.
Player: See, I've done the whole short arse thing before and I'm not really into it. Just bugs the crap out of me and I've done it before.
DM: Hrmm... Well, a Human from one of the further out places would work as well. Maybe a (insert bit of Tolkien stuff here, cos, well, I dunno but I'm sure there's something that fits the bill). That would fit your concept pretty well and they're known for being brutish warriors and totally uncouth too.
Player: Hey, that sounds cool...
See, both sides were willing to compromise, although the DM was going to stand fast on the no orc thing. They asked each other questions and got to the heart of the matter and everyone walks away happy.
To me, this is how this conversation should
not go:
Player: Mr. DM, I've got this concept that I'd really like to play in your game. I want to play an Orc.
DM: What? I said no orcs. It's in the handout what you can play.
Player: But, I have this cool concept. What's the problem with an orc?
DM: Look, I don't have time to answer any of your questions. Get with the program and make something from the handout.
Player: ... uhh ok...
Meanwhile, the DM has now labeled the player as a "bad player" and will do everything in his power to block everything the player tries until the player finally quits in frustration.
Apparently, this is good DMing to some people in this thread.
So, to bring this back around to your quote Lamia, I agree that if the conversation devolves into an argument that lasts for half and hour, then totally, there are some serious problems at the table. But, it should never get to that point.
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Just wanted to add another thought too, on the idea of the DM is Law. My current group consists of six players. Between the six of us, we've got somewhere around a century of gaming experience. Every single one of us can run a good game and I know that for a fact. Heck, I'm actually a bit intimidated to be honest. These guys know their poop.
So, who am I to just shut down conversation and decide that I know better than they do what would constitute a good game?
Imagine for a second that myself, Raven Crowking, Lanefan, The Shaman and Pemerton sat down at the table to play (ignoring for a second the blast damage as anti-particles collide

) and I'm DMing. Every person I just listed has been playing for many years and knows their stuff. I know every one of them runs a good game. Isn't it the height of arrogance for me to just flat out state that I know what would be better for this campaign than any one of them?
See, I trust my players. I really do. I know that they are every bit as invested in making sure that the game goes well as I am. I know that if we bring something in that doesn't work out, getting it out of the game will be nothing more than a short conversation. Heck, I've dropped things into my game that I didn't think were overpowered and they came to me to tone things down.
Did I mention I love my group to pieces?
My advice, and I'll stand by this, is if you're a DM, relax a bit. Let the players make some changes. You'll be very pleasantly surprised by the results. The players will be more engaged in the campaign because they can claim a bit of ownership and responsibility over the campaign. And, the more engaged your players are, the better your campaign will be.
Not so long ago, I'd be right behind Zel, and the rest saying the DM's word was law. Then, I had a bit of an epiphany a few years back and learned to relax a lot. It has done nothing but make my games better.