Perhaps, the most significant one is the addition of weather, as opposed to climate, which was already there. Now, there is a chance that storms will develop over the battle field. These storms play the following roles:
1) Affect air operations.
2) Help cause the transition between the seasons.
2) Hide ships during storms at sea.
Now, air operations are allowed during the "mud" seasons, but a "storm" hex impedes air movement, and they cannot be conducted in or through a "storm" hex. The lone exception, which I don't think can be stopped is if an airbase is in a "storm" hex, but has non "storm" hexes adjacent to it, the air unit based there can conduct an air operation and successfully RTB.
This is what storms during summer months look like:









