Image source: Getty Images. Key Points Over the past decade, the S&P 500 (SNPINDEX: ^GSPC) has been on a terrific run. The benchmark large-cap index has generated a total return of 300% (as of Friday). That gave it a compound annual growth rate of about 14.9% -- meaningfully higher than its long-run average of about 10%. Investors have reaped the rewards. However, the market appears to be heading into a less common phase, and it would be a good idea for investors to pay close attention. Looking at those pat ...