Mostly single-hand maneuverability of device, is what I find most useful.

I'm pretty dexterous person with medium-sized hands, and weight and form factor of 4S allows me to easily fetch it from any side, flip it, access any button, and manipulate any point on screen while still keeping weight center in palm of my hand, not fingers or, god-forbid, fingertips.

I sort of could work with Samsung's SII, but SIII I couldn't tolerate.