Really, one of the biggest stumbling blocks for any design focused on simulation, that is meant to apply to a wide audience, is that it is hard not only to drift it out of simulation, but also harder to drift it to other simulation. Or at least it is once those other simulations become different enough to deserve that label.
GURPS probably pulls off that trick as well as anyone, and it still feels like "GURPS with magic" or "GURPS with laser rifles" or "GURPS with fluffy, psionic rabbits" no matter how much you change the simulation.
This is a fundamental problem that D&D has fought with since the first new class was introduced and any kind of option discussed. Take, for example, the fighter class, and at 4th level the fighter gains some kind of ability. If you limit it to one particular ability, you are making a strong statement about the world being simulated. You can try to make that ability broadly useful and characteristic to compensate, but you can't entirely avoid that strong statement. OTOH, if you let the fighter pick from a list of three to five abilities, you have now, inevitably, watered down each possible simulation just a little. (Well, maybe not with the fighter, because some good choices can be made here. But across all classes and levels, you'll have some mistakes.)
Pull those abilities out into "feats" or "powers" or "magic items appropriate for about 4th level" or any number of things, and you'll become more flavorful, but inevitably lock the simulation down even more.
Thus, as was stated by someone a few weeks back, the answer that often gets suggested is, "Make a fighter class with few or even no choices that exactly models what I want in my game." The designers can't do that.
