The weather.

[Gregoshi]

Regarding weather and the seasons, I think we will be getting our wish. Here is the part of the IGN.com preview that talked about seasons and the weather (the bolding is mine for emphasis):

The art team has been inspired by all of the conventional icons of the land and the wide range of seasons and weather will help add variety to the battles. You may, for instance, find yourself fighting in the same province during the spring as cherry blossoms drift by, and then come back for a battle as layers of snow sit on the land in winter. The team is reintroducing nighttime battles to the game, and we saw a spectacular example of a castle assault during a thunderstorm at night. The way the lightning cracked in the distance and lit up the trees and soldiers as rain collected in puddles around them was phenomenal.

The only small leap of faith (i.e., assumption) is that weather/season will actually influence combat instead of just being eye candy. You may commence celebrating this in your own personal way.