To be honest, I don't really need more CPU or GPU power (though of course it's always welcome).
Longer battery life is always a plus too.
USB 3.0 isn't a big deal to me, since I seldom use any USB devices, nor even own a USB 3.0 device.
HD FaceTime camera would be nice, but I don't FaceTime all that often anymore.
SATA III SSD will of course be awesome, which I expect to see (I'm pretty sure the current ones are only SATA II?)
If I had to pick anything, it would be the foot print. My 2009 Sony Z was smaller in length and width, and had a 13.3in screen with a higher resolution. I miss that. It was close to the size of the 11in MBA. It also had a dedicated GPU and carbon fiber lid. MmmMmmm
If they could just fit the 13.3in screen into the 11in chassis, I would legitimately run into an Apple store saying 'shutup and take my money'
EDIT - A close second would be 8GB of RAM. Duh! How could I forget that.