If I recall correctly, mud WAS going to be in the initial game, but they discarded it as bad for gameplay.
I remember someone at Relic (possibly QD) stating some time ago, that they were thinking about implementing this kind of feature but they run into some technical issues that they found difficult to overcome so they dropped the idea (apparently not).
I'm not thrilled by this, because heavy snow is the thing I hate the most from cold-tech... I can even live with blizzards but deep snow is making flanking so ridiculously harder, which is totally bad for this game.