The role of the GM and character conflict...
Hearkening back to an earlier post regarding the BlackMoria situation: *if* the player of the elf *knew* about the culture of dwarves (unclear from BlackMoria's telling of the story), and *if* the player *knew*the personality quirks of the character BlackMoria was playing (far more likely), then the player was intentionally setting up a conflict. The GM's responsibility at that point *if* he'd wanted to avoid the conflict would have been to tell the player of the elf, "No, you can't play a dwarf-hater. I don't want the contention between you and the dwarf to become the focus of the party's dynamic."
Taking it a step further, it is rather clear from most of the subsequent posts that particular groups and individuals can go one way or another with regard to the type of character conflict that is acceptable within a gaming group and/or a campaign. These seem to range from the level of "comedic rivalry" to "burgeoning murder".
Players set up those kinds of inter-party conflicts only with the tacit permission of the GM. The GM is responsible for assisting the players in keeping the game fun and entertaining for *everyone.* Some GMs find high levels of conflict entertaining because they don't have to do a lot of work and it's like watching a TV show. Some groups don't mind high levels of conflict between characters because they switch out characters like clones in the Paranoia system. For campaigns of an epic scale, conflict between characters can render them the status of archetypes for the opposing polarities of the main source of conflict in the campaign proper. Conflicts *can* be used to further define character personalities, but I'm sure most will agree that the campaign isn't about defining character personalities alone.
Imagine a GM gathering a group of players together. What are the logical assumptions?
1) These players are going to create characters to play in a role-playing game.
2) The GM is responsible for what the characters perceive from the campaign world, so in effect, he's responsible for the campaign stories and plotlines in which the characters become involved.
At no time should anyone assume that players will automatically create characters that will, in character, get along unless the GM specifies that it is a condition of the campaign. Some might assume that it's a granted, but different styles of role-play don't automatically take it into consideration.
Now, from what I've read of the previous posts, most people game only with friends. Being friends assumes at least on the surface that each person shares something in common with the rest which results in a feeling of common affinity. It's only human to over time expand that feeling of common affinity to assuming that your friend shares more than maybe one thing in common with you, even though it's really never come out in conversation. In this case, all the players assume that each individual is aware of a certain etiquette of gaming, and that they all game for the same reasons, and get the same type of entertainment from gaming.
So Player A might assume that Players B, C, and D game the same way he does: creates a character that will 1) fill a niche in the party, 2) accept at face value any adventure thread that comes along, 3) will fulfill the "duties" required of the niche he has filled, i.e., cleric=medic, and 4) suborn the character concept to keep harmony in the group. This player enjoys the archetypes and conventions of DnD and is a solid team player.
In the meantime, Player B creates a character that might happen to fit a niche, but in game constantly questions any plotline thrown to the party by the GM in the interest of realistic portrayal of his character, doesn't feel obliged to fulfill any "duties" of his character profession "just because", (i.e., out of obligation to the conventions of gaming) , and feels that he is holding true to his character in an effort to get everyone else to play realistically and in character. From his viewpoint, the conventions of DnD are not what he is playing for, and he expects his friend Player A to accept that, not to mention the rest of the group.
Neither of these players are "bad" role-players. However, friends or not, their approaches will in fact create conflicts if *no one* in the group takes the time to outline the differences and potential conflicts inherent in their different styles to bring them to a compromise, if either or both are amenable.
It's also important to remember that while imitation is the sincerest form of flattery, most gamers nowadays don't jump into a RPG campaign intending to recreate the depth of genuine camaraderie found among Tolkien's literary characters in the Fellowship of the Ring (Lord of the Rings), where a player like Player B would thrive because his character would be given *reasons* to do the things expected of him in the adventuring party, like friendship, loyalty, trust, etc.
To reiterate what others have said on the board--It is important in the end that the GM take some measure of responsibility in heading off the conflicts with the potential to blow his game to smithereens, even if it is to vindicate different styles of play, or specify the limits to which conflicts can be taken.
I assassinated (planned and planned and planned) the higher level holder of the Hand. Somehow, he lived and I ended up in bondage to some demon or somesuch! Never got back to that campaign after that before we all started moving on and moving away. Pity, it was a blast! Had nothing to do with hate, had to do with power. All of us were willing to do to our companions whatever it took to gain that power as long as we didn't get caught,,,until it was too late! Delicious! 
