It's amazing how symmetrical the market can be over time.  I like to call it the "Pendulum Effect*".  Typically when a market swings in one direction, once it reverses, you can expect a counter-reaction of a similar degree (like a pendulum).  There's no better example of the pendulum effect than the price action within the equity markets over the past two years.  True to form, the sectors that were hit hardest during the bear market have "swung" back sharply to perform best off the March 2009 lows.  Meanwhile the areas that had modest declines in the bear market have had modest gains in the bull market. 

The attached charts bring home the point very clearly.  The pendulum effect underscores the importance of active management (as opposed to a passive, buy and hold strategy).  Over time, trends come in and out of favor and a good active manager can identify market trends (and trend changes) and capitalize on them to enhance portfolio returns.  By simply buying and holding for the long run, you are inevitably going to experience both swings of the pendulum (the good and the bad).  The goal of any active manager will be to keep you on the winning side of the pendulum at all times. 
 

*For the record, Bill prefers to call it "The Elasticity of Returns", comparing the price action to an overstretched rubber band.  The more you stretch it (as long as it doesn't break), the more powerful the reaction.   Take your pick....I'm sticking with the pendulum!