For purposes of D&D, what's the difference between a Norman knight, a viking, a samurai, a Roman centurion? What if we throw a muskeeter in the mix, a Mongolian horse archer, and a Hospitaler?
Is there something that prevents a player from being "knightly" because both the knight and the viking are represented by the fighter class?
I love this question with regards to the fighter, because the convergence of technology and tactics has created great variations in the fighting men of world history, and the differences are so easy for us to visualize. The evolution of D&D from its original vision has involved a long line of add-ons and extensions to create mechanical differentiation among character classes - new classes and sub-classes, secondary skills, kits, feats, prestige classes, and so on.
It's a topical question for me - I'm thinking over what a game in an Oriental Adventures setting would look like using a BX style of rules, and I have a pretty good idea how I'd handle archetypes like the samurai, the bushi, the kensai, the warrior monk: you're all fighters. You wear different armor, use different weapons, but at the end of the day, you all get paid to slug it out with the other guy. My work here is done.
And yet... if we run an Asian themed game using LOTFP, it has that nifty d6-based skill system, it'd be so easy to add some skills to reinforce the in-game flavor. If we ran it with ACKS, the upcoming player's guide is filled with class-building guidelines; one could probably convert the contents of Oriental Adventures to classes balanced with ACKS. It's so easy to start sliding down that slope - "story elements should be supported mechanically so players feel like their character can do something different or exclusive".
I'm just using the fighter as an accessible example, you can do the same thing with every class. Is the illusionist necessary? Isn't an assassin just a guy that kills people for money? Does the wu-jen really need a separate spell list? And so on.
Seems like a good time for a new poll: How important are game mechanics versus flavor?