Chart(s) of the day...how this dropoff compares to the 30's. Normalized to 100 at the start of the recession.
Figure 1. World Industrial Output, Now vs Then
Figure 2. World Stock Markets, Now vs Then
Figure 3. The Volume of World Trade, Now vs Then
If you want to understand why it's important to take stimulative measures now, there's your answer. The good news is that at least a handful of people seem to have a better understanding of monetary and fiscal policy than was available in 1930-1931 when things were really contracting and when the response of the government was to allow the money supply to contract so that things could spiral even farther downwards.
Figure 5. Money Supplies, 19 Countries, Now vs Then
All of the x axes are time in months/years after the peak.