There's a big story in Barron's this weekend titled The Worst of Times to Buy Stocks?, which highlights the gloomy warnings of well-known investor John Hussman and technical guy Walter J. Zimmermann Jr.
Hussman's bearishness is well known, but the article by Randall W. Forsyth boils down Hussman's bearishness to five criteria:

• the Standard & Poor's 500 trading at more than 8% above its 52-week exponential moving average

• the S&P 500 up more than 50% from its four-year low

• the "Shiller P/E," based on the cyclically adjusted trailing 10-year earnings, developed by Yale economist Robert Shiller, greater than 18; it's currently 22

• the 10-year Treasury yield higher than six months earlier

• the Investors Intelligence's bullish advisory sentiment over 47%, and bearishness under 25%; in the latest data, the numbers were 47.9% bulls and 26.6% bears

Apparently all those conditions are nearly in place now, as they were in 1987, 2000, and 2007.
Meanwhile, Zimmerman agrees with all that, plus he cites the inevitability of a market decline owing to rising taxes, austerity, too much bullishness, and gas prices. He sees a "perfect storm" manifesting itself within days.