winter damage/zones

Just wondering--I understand that a major determining factor of assigning  a growing zone is the lowest expected winter time temperature. In what different way is a plant affected when the low temperature is say, minus 10 F verses minus 20 F verses minus 30 F. It seems to me that at some point, warmer than the example temperatures, that plant tissue would be frozen completely and any lowering of the temperature would not make any difference. What happens?
 
Along these same lines, here in zone 4 I am used to things freezing up around December 1st and more or less staying that way until around March 1st. We can also expect  a protective layer of snow during that time.  The last three years we have had very little lasting snow cover  because of the many days above freezing and the accompanying warmer nights. I  have been following the advise for years to mulch certain plants in the late fall to avoid winter damage from freeze/thaw cycles. So my next question is: At what point can I expect damage  on the general plant population from all these  unusual freeze/thaw cycles?
 
Hank Zumach
Stoddard, WI
zone 4
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