Terrain should lose most of it's movement speed penalty when it's below freezing

[NWCtim]

Mud doesn't really slow you down when it's below freezing.  There would still be penalties from things like snow and plants.

This is more of a realism/immersion issue than a gameplay balance issue.

[akiceabear]

And lakes could freeze over! 

[Eleazar]

marshes look like the freeze over in the game.