While the sleek touchscreen defines the iPhone's design, one of the things I like about it are the simple mechanical buttons on the side. There's a dedicated volume rocker which instantly makes the iPhone a better music player than any iPod. But even better is the silent mode switch, an old fashioned mechanical two position switch. Slide it away, feel a satisfying click, and your phone is now in silent mode. There's even an orange dot visible for visual confirmation.
You can measure the disaster of cell phone UI by how many button presses it takes to silence the damn ringer. My first Nokia phone took 2, the Ericsson took 3, and on the RAZR it's like 17 button presses. You don't need silent mode often, but when you do you need it quickly and without a bunch of screen reading distraction. The physical switch for that is lovely.