The Absence of Abruptness

The absence of abruptness is a key element of good user interface as Kathy Sierra points out in iPhone and the Dog Ears User Experience Model.

Fluidity, follow-through and bounce.

Even if you don't notice it consciously, an animation (even of just words) feels more appealing and alive when things move in the virtual world more like things do in the real world (or even more exaggerated). It feels more lyrical, fluid... less abrupt. And that is what the iPhone UI does.
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...it wasn't the scrolling that made my jaw drop... it was what happened when the scrolling stopped: it bounced! The thing actually bounced if you flicked it hard and fast enough to send it flying up to the very (or bottom) of the list before it had a chance to slow down and stop. It actually bounced. And until you've seen it slow down and bounce, you haven't felt that visceral, life-like, fluidity.