The bear market of 2008 was a game-changer for many investors. Prior to 2008, a market decline of staggering proportions was a philosophical idea. The Great Depression was a distant event that few people alive today were even around to experience it - and most them were so young when it occurred that it had little or no impact on their personal investment portfolios. (Remember, the 401(k) wasn't even introduced until 1978, so even the Great Depression did little to derail the retirement dreams of the average investor.) Now that we've lived through a stock market decline in 2008-2009 that not only wiped out a decade's worth of growth but also changed the face of Wall Street forever, what have we learned? Here we look at the top lessons.