It's an old market adage you've heard before: Bull markets climb a "wall of worry."
That wall was particularly high last year, when the S&P 500 vaulted to a 30 percent gain. New worries seemed to crop up constantly, with traders fretting about everything from congressional budget wrangling to the sputtering global economy, and yet stocks soared.
The new year has been a bit bumpy for stocks, and analysts note far fewer worries—but the wall still holds some key bricks.
"The consensus is beginning to believe that we have this synchronized global economic expansion, and therefore I think the wall of worry is actually decaying," said Mark Luschini, chief investment strategist at Janney Montgomery.
He does not expect the big gains of 2013 but does foresee an 8 percent or 9 percent rise in stock prices this year.
Apparently, the more furrowed brows, the better the potential gains. The phrase "wall of worry" refers to the market's ability to continue to climb higher in the face of headwinds.
Just take a look at how far the market has come since the depths of the financial crisis. Remember that day back on March 6, 2009, when the S&P 500 hit an intraday low of 666?
We're back up to 1,835. And during this entire massive bull run to the upside, there have been plenty of stumbling blocks—economic, political or otherwise. All those problems can be thought of as bricks that go into building the wall of worry.
Here's what traders, investors, strategists and analysts say are some of the biggest worries for 2014:
Investors say on element in the wall of worry is Fed policy and the rising interest rate environment. With the central bank's December decision to begin tapering its bond-purchasing program, BTIG Chief Global Strategist Dan Greenhaus acknowledged the risk that the Fed could adjust its policies too quickly or too slowly.
Steven Ricchiuto, chief U.S. economist at Mizuho Securities, said that his top worry for the market is that Fed tapering could lead to rising rates, which could in turn thwart the economic momentum from the second half of 2013.
"The Fed's QE program distorted financial markets and exiting could create vacuum of sponsorship leading to a rapid sell off in belly of the curve," he said. "This countercyclical rise in rates would become an unanticipated headwind."
(Read more: Fed's Richard Fisher: Liquidity causing 'beer goggles')
Another bigger picture, or macro, concern is growing geopolitical risk, including turmoil in places such as Turkey and Thailand.
Those are two of the countries that Andy Busch is keeping an eye on.
The author and publisher of "The Busch Update" said that both markets saw considerable volatility heading into 2014. A corruption crisis surrounding Turkey's government helped push the Turkish lira down by about 17 percent last year.
Anti-government protests in Thailand have resulted in the parliament's being dissolved, and the country's benchmark stock index has lost 12 percent over the last three months. Busch said these uncertainties could lead investors to pull funds out of many emerging markets.
Busch is also focused on China, still considered by many to be an emerging market, even though it is the world's second-biggest economy.
Its sheer size means that any development there could have ripple effects all over the world. For example, signs of slower GDP growth, coupled with other fears about emerging markets and rising interest rates, could cause a sizable disruption in financial markets.
On the micro, or more company level, side, we're entering what could be another volatile couple of months as corporate earnings season gets underway. Fourth-quarter earnings season unofficially kicks off Tuesday with reports from JPMorgan Chase and Wells Fargo.
With a large number of warnings for the period, this earnings season could create catalysts for the market. Traders are watching each report carefully for themes and data points that reveal the outlook for the year.
Cliff Noreen of Babson Capital, who runs about $188 billion in fixed-income assets for pension funds and endowments, is concerned that weaker corporate profits could weaken the bull run. He thinks corporate expense cuts and worker productivity have likely peaked, which means any profit growth would have to come from fundamental improvement in sales.
(Read more: Earnings pose the next hurdle for stocks)
Add to those concerns over U.S. unemployment, stock market valuations and the threat of more violence in Russia leading up to the Winter Olympic Games.
The question becomes whether or not, in the words of Pink Floyd, each is "just another brick in the wall?" And even if the wall gets harder and harder to climb, traders wonder whether the Fed and other central banks would let markets decline in a disorderly fashion.
No matter what, though, worries are there. And if history is any guide, they probably always will be.
—By CNBC's Dominic Chu and Elizabeth Schulze. Follow them on Twitter @thedomino and @eschulze9.