Well, putting aside the question of whether or not there should be multiclassing (that's another discussion), this is where the idea of Primes and the like comes in: you get your character differentiation by what your character's secondary (and perhaps tiertiary) abilities are. That is, if you're a Fighter, you're assumed to be Strong (Strength Prime) but perhaps you choose two secondary abilities and three tiertiary (for example). What are they? Alternately, what if your fighter chooses the Aspect "Cunning" as opposed to "Charismatic" (or, for that matter, "Arcane Dabbler") with your chosen aspecting givine you a pre-defined bonus/modifier.

Funny thing is, D&D 4e multiclassing seems aimed in this direction: preserving class roles a bit more rigidly, with just "a touch" of capability from another class rather than something closer to a 50/50 blending of classes.