Are investors ignoring risk?

Volatility (measured by VIX) is at an all time low, the stock market at an all time high.  VIX measures the volatility implicit in the futures contracts that investor's buy.  It can be thought of as the price of insurance against a stock market plunge.  Right now, investors are not willing to pay much for insurance because they think the probability of a plunge is small.

What could go wrong?

To see the answer to this, look at early 2007, when volatility dropped to about the same level.