From Fidelity (11/25/2008), see chart above (click to enlarge) of the 5 reasons; here's the summary:
The challenges faced today by the global economy and financial system are staggering. For the United States, all economic indicators point to an economic downturn that will at least rival any in the post-war period. However, all historical analogies are imperfect. The world changes too much over periods of decades, particularly economies powered by constantly changing technologies, to find a precise fit for any historical parallel. While there are admittedly some similarities between today’s environment and the 1930s, those similarities do not mandate that the world is predestined to follow a path into a decade-long depression.
The dramatic response by central banks and governments around the world faces challenges and potential ill side-effects of its own. But this response underscores that we are living today in very different times than the world experienced 80 years ago, and it may serve investors well to take those differences into account.