When you look at a city from above, what do you notice first?

I've been sketching rooftops for a picture book about a bird's-eye journey, and I keep getting stuck on how much a city tells you from above that you never see from the street. For those of you who've looked down from a plane or a tall building — what's the first thing your eye lands on? The patterns of yards, the way light hits different roof angles, the strange geometry of parking lots?