None of my real estate is underwater. I have lost paper value on some, but I don't owe more than the market value on any.
Unless people are FORCED to move for some rare reason, primary residence homes being underwater doesn't really matter, right? What's the big deal?
Among other things, it becomes impossible to get a home equity loan. Traditionally, homeowners have borrowed against the value of their house--including taking out their built-up equity--for renovations and repairs, and to pay for life events like school and marriage. The existence and availability of home equity loans have been one of the drivers of the economy, partly because homeowners can effectively convert non-tax deductable loans into tax-deductable loans, freeing up money for other purposes. To give a simple example, when I bought my first house, it was appraised for more than we paid for it, so my wife immediately took out a home equity loan for the difference, and used it to pay off some student loans. As a result, we were able to both lower the interest rate on that debt, and it was now tax deductable because it was backed by the value of the house.
Also, the fact that it is now impossible to sell your house has a bigger impact than you imagine. You have the retiree who wants to downsize and save the costs of utilities, property taxes and upkeep, but can't. You also have the worker who wants to move to another city or state to take a better job but can't. Not only do they have to bring money to the table to sell their house, but often homes will sit on the market for months and months before selling without essentially giving it away. This, in turn, constrains the economy from growing, because available jobs go unfulfilled, and qualified workers can't take them, because the cost of moving is too high.
In three years I will almost certainly sell my house and move. I am much more concerned about it sitting on the market for a year than I am about selling it for a $50,000 or $100,000 loss.
One issue that is rarely addressed is that homeowners tend to think that it is their "right" to make a profit on their house, and see it as an investment. A house is not an investment. It is a home. You just hope that it retains its value. It is a fairly straightforward analysis to determine the break-even point between renting and buying. You look at your expected expenses, including upkeep and taxes, and the deductability of mortgage interest, the transaction cost (closing costs, broker fees) of buying and selling a home, etc. There are a lot of variables, but generally speaking the break-even point is four-and-a-half to five years. That is, if you live in a house for five years or longer, and can sell it for the very same amount that you paid for it, you will be better off than if you had rented a comparably-sized apartment over the same period. If you do the spreadsheet properly, you can conduct an intricate sensitivity analysis, and see how the break-even point would shift based on different assumptions. I would be thrilled to sell my current house for what I paid for it (plus capital improvements) when I sell it five years after I purchased it.