Keys rubbing against the surrounds
Not sure how common this is, but I tried the new top end Air in a 13" today and spent 4 hours after going over the display and other various parts to ensure I didn't find any defects. When I started to use it to type, I noticed the keys in the middle row are grinding towards the left side of the metal surround when being pressed at times. It gets more and more pronounced as you continue to type until it feels like the keys are binding. Otherwise, a good machine, I just hate wasting time on something so expensive. Going back to the apple store tomorrow and will probably just seek a refund.
iMac, OS X Mountain Lion (10.8.2), 27" Mid 2011 3.4 Ghz 16Gigs Ram