Lord Mhoram
Hero
6[MENTION=98255]Nemesis Destiny[/MENTION]
Ron Edwards gets this pretty right, I think, here:
Consider the behavioral parameters of a samurai player-character in Sorcerer and in GURPS. On paper the sheets look pretty similar: bushido all over the place, honorable, blah blah. But what does this mean in terms of player decisions and events during play? I suggest that in Sorcerer (Narrativist), the expectation is that the character will encounter functional limits of his or her behavioral profile, and eventually, will necessarily break one or more of the formal tenets as an expression of who he or she "is," or suffer for failing to do so. No one knows how, or which one, or in relation to which other characters; that's what play is for. I suggest that in GURPS (Simulationist), the expectation is that the behavioral profile sets the parameters within which the character reliably acts, especially in the crunch - in other words, it formalizes the role the character will play in the upcoming events. Breaking that role in a Sorcerer-esque fashion would, in this case, constitute something very like a breach of contract. . .
a character in Narrativist play is by definition a thematic time-bomb
I'd pretty much made this decision already, but thanks for reinforcing my decision to never play a narritivist game - assuming this is a pretty good description of it.

As for the general discussion - I'm going to throw in another idea - Immersionism. When playing any time I change the world, story, direction of narrative but do so outside the actions of the character (such as using a fate point, drama point, or even hero points as they show up in some D&D games) completely pulls me out of character and into metagame mode. The purpose of roleplaying for me is to become my character in play as close as possible - and any mechanic that affects that gets between become said character and me. Mechanical concerns such as hit bonus or skill bonus don't because there is an in character knowledge of skill - the character knows if they are good at lockpicking or such. Rolling the die is me making an action that is a 1 to 1 equivalent to the character picking the lock.
I know my playstyle is not all that common - but when narrative or personality mechanics come into play they destroy, by their very nature, the purpose of any roleplaying game.
I mostly talk about my tastes rather than in generalities for example, but when discussion reasons and modes of play that aspect is almost never brought up; and can have an incredible impact on game style, and game choice. I prefer D&D and Hero (even with disads, but they are, as Ron mention, to define the character) because they don't have rules for the kind of thing being discussed.