Disclaimer: I am not suggesting making a EU Campaign the standard game mode. I believe a campaign like this should be for organised collections of regiments, not casual randoms.
This is an argument in favour of the European Campaign, that I think would add both realism and fun over NW.
Imagine an average line battle. I think we can all agree that the most fun part is in the middle of the battle, when all the shooting and charging and meleeing is going on. On the other hand, the most long and boring part is the closing, when there are only half a dozen line infantry and artillery-men left. The all charge is ordered, and both sides have to run across the whole map to finally meet in the middle and fight, without shooting, until one team is completely annihilated. Not only is this unrealistic (who would fight to the last man?), but also boring and not very fun, both for the spectators and the people running across the map.
But, with a EU Campaign, it would be different. Fighting to the last man costs both sides huge amounts of manpower. And if there were a system that would "de-activate" a regiment for 1 turn if they had more than, say, 80% casualties, it would had a real tactical interest in retreating, routing or surrendering.
If the commander of the battle saw that the odds were against him, he could retreat to salvage as many units as possible and save manpower. If a regiment is at 50% casualties and know it will go over 80% if it charges again, the leader can order a retreat or a route to avoid being deactived. If it are surrounded or about to go over 80%, the regiment can surrender, transferring manpower to the ennemy but avoiding the 1 turn cooldown.
This solution would make the game a lot more realistic, something that everyone here seem to embrace dearly. It would also remove the boring "all charge", turning most of the battle into the fun "shoot & stab" part.