I think it's our clean air and fresh Rocky Mountain Spring water !!
Seriously, over the years I have found a number of business strikes from the Denver mint that have mirror-like surfaces. I always have to look twice to make sure these are not proof issues that were put into circulation.
All of the recent D mint pieces I have in my collection are pulled from circulation, and many of them are gems! Unfortunatley, I usually don't start seeing coins from Philly showing up here in Colorado until late in the year, and these are usually fairly battle worn by the time they circulated across the Mighty Mississip. I'm sure you folks in the Eastern US experience the same thing with Denver pieces.