One of the great strengths of BoA is how well you have captured the seasonal nature of 18th century warfare. An army that doesn't go into winter quarters in the north will pay the price, as its troops desert or die of exposure.
But I wonder if you don't have too much of a good thing. It is not uncommon in the game for rivers and lakes to stay frozen until May, or to encounter harsh weather and snow in June at the higher elevations. Now, our Canadian winters may be brutal, but they are not THAT brutal. Spring thaw comes in March, and by April the snow and ice are gone.
On the other hand, movement penalties during "mud" turns are perhaps not severe enough. Even in places where roads existed, they weren't much more than cart ruts. Mud would nearly immobilize an army (although perhaps not indians or irregualrs). Supply wagons and artillery would be difficult to move at all.
Prehaps in a future patch you could limit the winter effects to November-March, but strength the mud penalties in April and May.