Not just a visual winter, and not a Aquilo-style freeze - dynamic seasons come to Nauvis with moving snow and thaw fronts, freezing lakes, increased factory energy consumption, heating, and mineable ice
Mods introducing new content into the game.
Map generation and terrain modification.
New Ores and resources as well as machines.
Changes to power production and distribution.
dynamic range version.
Dynamic rain*
It's already optional. What happens when you try to use Winter is Coming without it?