Having some differences between the various nationalities could be done and is, I feel, a very valid suggestion. The precedent has, of course, been already set in other wargames, both board and computer. Now, I'm not suggesting that you need to hardcode the morale values of a side; the way the game models that now is fine (where the units can range from conscript to elite). The scenario designer (or the person purchasing his units in a DYO) can get the quality type that is realistic to the situation.
What could be done, however, is to try to model some specific *trait* that a nationality is perceived to have had. For instance, (as has been pointed out) perhaps US units should rally faster than others. One way this could be modeled is by the program giving the Amis more leaders with the rally bonus "heart" -- or hard code it otherwise so that recover from Panic status faster. Maybe some British troops should be stealthier than normal? Give more leaders with the Stealth bonus -- or internally tweek the numbers that govern steathiness & becoming spotted. Perhaps German troops should have better attack power. Again, the game could award certain German troops types more leaders with an Attack bonus. Maybe this could only be for June-August of 1944...
Too, a bonus could be given to (dreaming here...) Russian troops to allow them to dig in faster (oh -- but I guess we first need to actually be able to dig in -- but that's another request!).
So, trying to model nationality-specific traits could, I think, be done -- *if* the designers of course really wanted to (and had time to program it). In fact, such a "rule" could be turned "on" or "off" (if all players in a multi-player game didn't agree to it) by it being optionally available.
One thing to keep in mind though -- positive benefits or traits should be balanced out by negative benefits or gamers will be very loath to play a nationality that has all-bad traits.
just my thoughts,
charlie