The most important ones are rifle ranges (200 yds); road march rate (about 30% greater); canister (200 yds); in GCM random maps fences and buildings are removed (to improve performance); potential addition of objective holder officers (otherwise, non-functional) & objective holder wagons (do not have supple capability); improved toolbar; modified impact of flank fire and rear fire on combat; graduated elevation bonus; modified recovery rates after unit breaks; artillery is, overall, somewhat less effective/devastating; guns recover fatigue more quickly but incur fatigue at a greater rate when using recoil/retreat; cavalry effectiveness is significantly reduced.
I'm sure there are many more, but these are the ones that come to mind.