What makes one home's value plummet when a similar home in a similar neighborhood remains stable? That is the million dollar question in real estate. Data show that home values can vary depending on the neighborhood, the age of the home, location/lot desirability and the economic stability of neighbors. No neighborhood is immune to this housing downturn, but some neighborhoods are suffering more than others.
In general, areas that had minimal decreases have been around for some time and were not as susceptible to investors. Even in those neighborhoods, however, there are exceptions. 
In many neighborhoods also the less expensive homes are hit the hardest. It's not just the age of the neighborhoods that contributes to value changes. It is also relatively low prices in certain areas that attracted investors. Like every neighborhood though, it all boils down to when you purchased. If you bought during the housing boom in 2005 and 2006, you're likely upside down and owe more than your home is worth. If you bought before then, you may still have some equity.