Air Power.
I know it's mostly an after thought and not really the focus of PC but I really like combined arms game play so I figured I'd ask the experts
I'm by no means an expert, but I was trying my hand at Sicily '43 Dog's Breakfast Campaign vs the AI. My Allied troops were constantly being strafed while in travel mode. Reading the designer's notes, I found out that in the Sicily campaign, it was mainly a case of friendly fire. I remember reading the U.S. troops referred to the USAAF as the "American Luftwaffe."
It had such an impact, that after so many losses of this type, I would rarely use travel mode in daylight.
Normandy '44 springs to mind. The Germans can only use the roads during the day at great risk.