After installing ML, I had noticed that when shutting down my 2012 MBA, it was taking longer than usual, and the grey screen with the spinner was showing up. Typically it didn't show up at all and the computer would shutdown within 5 seconds of confirmation. Now it was taking around 15 seconds. I had a hunch that this slowdown had to do with PowerNap. After disabling PowerNap, my shutdown times are back to normal. Obviously this isn't a huge issue, but if anyone else has noticed this, and was bothered by it, turning off power nap in the energy saver pref pane does the job.