well, do you want your characters to still be the focus of the battles or would you preffer troop numbers to rule the day?
I'd want a large force to generally beat a smaller force, but that doesn't mean the PCs shouldn't be the center of attention while playing out the battle.
Ideally the vast bulk of the combat could be played out with a much simpler system, while the key clashes involving our heroes play out in full detail.
are there other concerns you have had with other systems that you want to voice?
If the system is supposed to be D&D mass combat, it should play out like D&D combat writ large. That is, the characters' capabilities should be the same in the simple, mass-combat game as in the detailed, small-party game. Nothing's more annoying than getting killed by something you should be immune to (or whatever) because you're using different rules.
how well would you like high level characters to fare as solo entities on the battlefield?
They should fare as well as epic heroes. They should only die to other heroes or while making a last stand, etc., and they should inspire their own troops at least as much as they cut down enemies first-hand.
how simple vs realistic do you want the system to be?
If it's meant to be a D&D mass combat system, it shouldn't aim to be any more realistic than D&D. And it should be simple.