(Read more: Reluctantly, market faces a real default threat)
1. Depression and unemployment
Financial shockwaves, beginning at the Treasury and Federal Reserve, would make their way through banks and eventually blow a hole through the Main Street economy. Just as in the 2008 financial crisis, businesses would quit hiring amid the uncertainty. The unemployment rate would rise from its current 7.3 percent.
As an illustration, the jobless rate was 5.0 percent in December 2007, about where it had been for the previous 30 months, according to the Labor Department. By the time the Great Recession ended, it was at 9.5 percent, and peaked at 10.0 percent in October 2009.
(Read more: First a default, then a depression? Some think so)
A slew of other events would slam the economy: A drop in stock market prices, hurting many Americans' 401(k) investments; the seizing up of bank lending; and the U.S. losing standing in the international marketplace. With U.S. economic growth still below 3 percent, it wouldn't take that much to send the nation into a financial tailspin.
2. Dollar down, prices and rates up
Among the biggest impacts could be mass selling of the U.S. dollar, an event that would threaten the greenback's standing as the world's reserve currency.
That would pound consumers' buying power by boosting prices for everything from groceries to clothing to the gas we pump into our cars.
"In the event of an actual default, Treasury yields and other borrowing costs would probably rise and remain higher," warned Julian Jessop, Capital's chief global economist.
So homeowners and prospective homeowners would have to say goodbye to the low mortgage rates they have enjoyed while the Federal Reserve has kept its foot on the economy's gas pedal.
"All the money you're gonna have is under your pillow, and it probably won't be worth as much as it is today," Kyle Bass of Hayman Capital Management told CNBC's Squawk on the Street. "But I don't think we're going to get to that apoplectic point in the U.S."
With economic growth still below 3 percent, it wouldn't take that much to send the nation into a financial tailspin.
3. Down go your investments
Stocks have had a rough week, with the S&P 500 and Dow industrials off about 2 percent each and the Nasdaq down nearly 4 percent. That raises worries for many Americans whose nest-eggs are held in company 401(k)s and other retirement accounts.
During the last financial crisis in 2008, major U.S. equity indexes tumbled, with the S&P 500 Index losing 37 percent for the year, which translated into big losses for many 401(k) retirement plan assets, according to the Employee Benefit Research Institute.
Just how individual 401(k) participants were affected by the downturn largely depended on the mix of assets in their funds. For example, investors with a high percentage of their 401(k) in stocks (versus bonds or cash) took a bigger hit than those with more balanced funds.
While many analysts have been trumpeting the market's refusal to panic over the prospect of a default, that relatively sanguine reaction likely would change.
Estimates among Wall Street analysts are the market would drop between 10 percent and 20 percent — with the upper end at what Wall Street defines as a bear market.
4. Social Security payments halt
The current projection for the government to run out of money to pay its daily bills is Oct. 17. Economists believe, though, that the Treasury would have enough money on hand to pay its $12 billion Social Security payment due that day, as well as another one on Oct. 25.
That may not be the case come Nov. 1, though, when there's a $25 billion payment due, meaning that checks may not get issued past that date.
Nov. 15 stands as a larger date overall when the Treasury won't be able to make a $30 billion debt payment.
"We strongly suspect the current impasse over spending and the debt ceiling will have been resolved well before then," Capital Economics said in a report. "There is also a chance if the shutdown was still in effect at that point then the Treasury, perhaps with the Federal Reserve's help, would be able to avoid a default somehow. But in a worst case scenario, this is the date to watch."
(Read more: So what if there's a default? 5 things to remember)