Wikipedia has this nice chart, among others, here:
The percentage loss was big, but won't be truly significant unless it becomes part of a larger pattern of losses like we had in the fall of 2008, which dominates this chart.
For all we know, the decline today and Tuesday was the market's verdict on President Obama's and Senator Harry Reid's expressed resolve to raise taxes in the very near future in the wake of the debt ceiling deal.
Notice, however, that the historic market lows of February and March 2003, and March 2009 are absent from this chart. Those were incredible buying opportunities which did not reveal themselves precipitously. Those lows were achieved by grinding down to them.
