What the central banks can't do is substitute for politicians dealing with budgets and structural challenges, such as delivering a fiscal union in the euro area or tackling the more than $600 billion in U.S. spending cuts and tax increases that will start in January unless Congress acts, Fels said.

Government Action

"Central banks have steered us from recession, but on their own they can't bring us into sustainable recovery," he said. "To get out of the twilight zone takes government action."

At Credit Suisse, Garthwaite says quantitative easing makes it easier for governments to cut debt by fanning inflation expectations and also boosts growth by keeping bond yields low.

Goldman Sachs Asset Management Chairman Jim O'Neill, who says he remains "one of the minority bulls," points out U.S. households are taking advantage of record-low interest rates to pare debt and home prices are displaying signs of a bounce. The ECB has committed to keeping the euro alive, and even some troubled nations, such as Spain, have turned more competitive. Meantime, China is transitioning to "higher quality" expansion based on local consumption, he said in an interview. "The likelihood of another global recession remains currently low."

'Feed the Market'

Mark Mobius, who helps manage more than $40 billion as executive chairman of Templeton Emerging Markets Group, said the Fed will continue to "feed the market" until employment -- stalled above 8 percent in the U.S. since February 2009 -- rebounds, and the ECB and Bank of Japan will join it in pumping out cash.

This will be "very, very" good for stocks and emerging markets, he said in an e-mail to Bloomberg News.

Economists at the Jerome Levy Forecasting Center in Mount Kisco, New York, are more bearish. Easier monetary and fiscal policies can deliver only a "contained depression" by helping offset the financial volatility and balance-sheet repair that would otherwise spell a deeper slump, they say.

"There's going to be fragile growth globally and a potential for instability," said Srinivas Thiruvadanthai, director of research at the center. The need for investors to stay "defensive" means they should buy U.S. Treasury securities, even with the yield on the 10-year bond at 1.84 percent yesterday, he said.

Precarious Economies

While few analysts are forecasting a return to worldwide recession -- and certainly no slump akin to the 0.6 percent contraction of 2009 -- the longer the expansion remains lackluster, the more precarious economies become, Saumil Parikh, a managing director at Newport Beach, California-based Pimco, said in a Sept. 12 report.