Hmmm... I can see where you guys are coming from on this, but I also think that we might be expecting too much variety...

You have to remember that while medieval armies would certainly have looked rather ad hoc, a general basic appearance still had to be maintained in order to keep oneself oriented in battle. Wearing heraldric patterns as identification doesn't work if everyone comes up with their own. In the end, it didn't really matter what you wore so long as you kinda looked like everybody else - so a group of 3 Brits might be wearing, say, a yellow lion rampant on a red background, a white rose on a red background, and yellow-and-red-checks, and while when you just look at the three of them they look like they're all on their own, when you put them in a large battle where they're fighting those evil French in their blue uniforms then they're suddenly all for the same team.

If you start actively looking for the individual differences among soldiers in a group, then you may find yourself going "Hey, these guys look the same" - but first off 1) there's almost ALWAYS something different, usually the shield and 2) when you watch your knights charge the enemy or your infantry in a melee suddenly the differences become vastly more apparent as compared to RTW.