Reading Levels at Amazon
It is a good bit of fun to compare the stats provided by the ESV blog with those found here. Just as you don't want to read only one Bible translation, you also don't want to read only one set of reading level statistics.
On the ESV chart, which displays statistics generated by Amazon.com, the order of reading level from high to low is NRSV, ESV, KJV. On the chart supplied at Christianbook.com the order is KJV, NRSV, ESV.