One is that the VIX index - a measure of implied volatility on S&P 500 options - is low. At 13.4 percentage points, it is close to its lowest level since June 2007, which is a sign that investors are less fearful than they have been at any time in the last five years.
This might mean they are too complacent. In the past, a low VIX has led to falling share prices. In July 2011 and April this year the VIX dropped - and stock markets fell sharply in the following month.