User blog comment:Snes/A Rework of the CDR System/@comment-69.141.8.150-20120619164931/@comment-69.141.8.150-20120730193047

"Armor and MR have no decreasing returns at all."

http://i2.kym-cdn.com/entries/icons/original/000/005/545/OpoQQ.jpg

The more Armor you stack, the smaller the mitigation return you get from future armor purchases. Look up damage mitigation values from going to 50 armor to 100, then 100 to 150, then 150 to 200. It doesn't scale linearly, it scales multiplicatively. Hence, diminishing returns or DR.