"I still believe what's happening to oil is related to there being too much supply, but the sell-off is sending ripples through the market about global economic growth," said Sonders of Charles Schwab.
Europe's economy was in recovery mode the first half of 2014 before being derailed by the Russia-Ukraine crisis. The economic sanctions the European Union put on Russia for its invasion of Crimea impacted Germany's economy, Europe's largest, far more than originally anticipated. Since then, Europe has teetered on the brink of recession.
The Euro Stoxx 50 index, the European equivalent of the Dow Jones Industrial Average, is up only 1 percent in 2014 versus the 12 percent increase in the S&P 500.
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The European Central Bank has stepped in to help stimulate the region's economy and is expected to ramp up its efforts early next year. If the moves work, Europe will come off investors' worry lists. Until then, watch out.
If you're willing to take a more risky view, some strategists say buying into Europe before it recovers could provide an excellent return.
"There are low expectations for Europe's recovery, which means prices are low," Koesterich said. In Europe, investors are paying roughly $14 for every dollar of European company earnings compared with the $17.50 investors are paying for every dollar of U.S. earnings.
China and Japan are also concerns. The Chinese government is still in a multi-year effort to slow down the country's rapidly growing economy, which has had mixed results. Japan, meanwhile, is trying to stimulate its stagnant economy through massive amounts of economic stimulus. While the stimulus helped briefly, the Japanese government had to raise taxes to cover the cost of the program, effectively negating what the officials were trying to do.
Bonds will have a tough year ... or won't:
The biggest prediction of 2014 to fall flat on its face was that bonds would have a bad year in 2014. They didn't; in fact, they went in the opposite direction.
Instead of the 10-year U.S. Treasury note going from 2.97 percent at the beginning of the year up to 3.5 percent, as many predicted, the benchmark note was yielding 2.18 percent as of Dec. 22.
Many strategists readily admit they completely missed on their bond predictions. Nevertheless, many investors are doubling down on their bond yield forecasts for 2015, with some looking for the 10-year yield to reach 2.5 percent to 2.75 percent next year. They reason that the U.S. economy is improving and the Federal Reserve is expected to raise interest rates.