Believe it or not, it's the fan. The PR about the asymmetrical fan and a broad frequency spectrum sounds like BS, but it's not. You either never hear the fan at all, or if you do, it's _barely_ noticeable. Second on the list is battery life. It's a lot better. Probably related to better power management and the upgraded CPU. But I can actually get ~5 hours out of my new 11" MacBook Air. Previously I'd get 3.5 hours max. I have the maxed out configuration (although the SSD is 256).