thejeff wrote:"What is so awesome about it that it's worth reworking the campaign so that it really does fit?"
What needs to be reworked, 'though? The object of running a game is giving people a chance to share some fun for a couple of hours, not for one frustrated wanna-be author to sit four other people down and tell them a story, and refuse to allow anyone to play any character that I don't pre-approve as existing in this story I'm telling them.
There is no 'setting' outside of the group fun we are having. If the setting is the Forgotten Realms, and somebody wants to play a member of the Sueloise Brotherhood, or an Eberron Changeling, then, bang, so it happens. If we decide to play a vampire game, and one dude wants to play a werewolf, there are balance issues, because vampires are the ugly stepsisters of the World of Darkness, and either they need to be toned up (or at least have their non-daylight operations rule negated, so that the werewolf player doesn't spend half the game waiting for the sun to set and the vampire players don't spend half the game waiting for the werewolf's daytime adventures to end). 'Theme' is not an issue. We aren't a boy band, and everybody doesn't have to dance in lockstep. We're more like the Village People, one dude playing a cop does not preclude someone else playing a cowboy.
Again, I'm spoiled by superhero games, where the players can literally say 'I'm playing the avatar of an Egyptian god I just made up' and I, as GM, do not have to GAF that Egyptian gods may not have existed in the setting I'm using until this very second. It's the work of a second for me to say 'Okay.' It's not like I have to go invent an Egyptian pantheon to go along with that character origin, or rearrange any pre-existing gods I've got going on or anything.
Two separate but related things: I like to have stuff in the setting connect and make sense. If there's a race in the setting, I like to know something about its history and how it fits in with the other races and cultures. This helps me root the characters in the setting and develop plots for them to deal with. The Eberron Changelings would be fairly easy to work in to most settings, since they don't really seem to have much culture of their own. They blend in to others. So they're a mystery with a few references to past historical events. Fine. Eberron's Warforged would require much more effort to find them an origin and a place.
Second and more important: I don't run (or often play) sandboxes. Nor do I just run players through basically unconnected adventures that they get sent on or happen to hear about.
I'm also not a frustrated wanna-be author sitting four other people down and telling them a story, and refusing to allow anyone to play any character that I don't pre-approve as existing in this story I'm telling them.
There's a vast area in between sandbox and railroad, where there's a basic concept and there are bad guys who have plots going on that the PCs will wind up dealing with, but they still have freedom to respond to what the outside world is doing. But sometimes the premise of the game I'm interested in running requires limitations. Obviously if the players aren't interested in the game I'm proposing, then it's not going to happen, but they can't agree to play in it and then bring characters that don't fit. Maybe theme was a bad word choice, but I didn't mean the party has to have a theme, like a boy band, but that the campaign itself has a theme or a concept. If the concept is elven court politics, that limits characters in one way. If the concept is "urban adventures of a gang of thieves", that limits the characters in a different direction. And that's where I'm talking about "What's so awesome about the character?" Don't come up with an excuse why your half-orc barbarian could be at the elven court, sell me on why this half-orc barbarian is going to make this elven court politics game so much better.
If your group is just a bunch of random murder-hobos who met in a bar and off killing things for fun and profit, then none of this matters. Include anything you like. I like my games to be more focused than that. I like that as a player too.
Oh, since you mentioned superhero games, not all concepts work there either. The one I mentioned where the GM went with it and it ruined the game was a superhero game. And partly my fault. The game proposal, as I heard it, was child proteges of 1950's superheroes. I played a demon mageling adopted and being taught by that world's Sorcerer Supreme equivalent. Another player was playing a ninja with some complicated background I don't remember. What the other players heard for the proposal and what the GM intended had much more of an emphasis on the 1950's 4-color, code-approved, straight-forward heroics. And that's what the other characters were. Clashed with our two gritty 90s style near anti-hero characters.
GM absolutely should have shot that character down. Didn't fit the theme.