On of the main selling points of the Nexus S is its "Contour" display. Google and Samsung claims that the Nexus S is the first device to have such a display, and they're right: think back to any phone you've ever owned and you'll recall that the touchscreen was flat.
What advantages can be had when your smartphone screen is more shallow in the center than it is at the top of bottom? First, it feels better against your face. Our faces are curved, and holding a flat slab to it isn't terribly comfortable. The curvature of the glass will hug the curvature of your cheek
thus allowing you to more quickly get cancer from cell phone radiation since you'll be holding the phone even closer to your head than you do now.
Second, the contour screen, coupled with a nicely placed ridge on the back of the phone, is going to make for a super comfortable typing experience. Your fingers will be able to rest at a more comfortable vertical distance since the keyboard will display where the contoured screen begins to dip. The ridge on the back will provide a secure resting space for your index and middle fingers.
Third, curved glass is unusual and interesting, which is going to make the Nexus S a lust-worthy device. Phones nowadays all look the same, but the Nexus S, from the across the room, isn't going to look like your father's Droid X.