The biggest effect was the transition from small, semi-professional armies to huge numbers of conscripted soldiers, known as the levee en masse. It was the French that really pioneered it during the French Revolutionary Wars. Since the Terror wiped out most of their office corps, the French army couldn't compete in the old style against their enemies, so, they called up a ton of conscripts (who often were motivated by the fire of the new ideas of nationalism), gave them weapons mass-produced in industrial factories, and sent them charging straight at the enemy to overwhelm them with elan and masses of skirmishing light infantry, which were relatively rare before this.

But really, I think the biggest change is in the number of soldiers involved. For comparison, the major battle of Rossbach during the Seven Years War in 1757, involved about ~65,000 soldiers. The Battle of the Nations in 1813, on the other hand, involved about ten times that amount, something like ~600,000 soldiers, and was the largest battle in European history up until WW1.