My experience in the snow was very positive. In my limited snow driving in California, I was able to go around other cars stuck in the snow with 21 inch wheels and continentals! From my experience, the S is probably better than most or all two wheel drive vehicles in the snow. Tesla's traction control is better!
I'm quite surprised when people are saying this. I have the best winter tires on my S (Nokians) and I found my S was below average in terms of traction in the snow.
Next winter, I'll make a video comparing my Model S, my Volt (with the same tires) and my BMW X1 (also on Nokians). I can tell you in advance that the Model S won't keep up with the Volt (and of course the BMW will leave both cars in the dust).