Materials

The iPhone 4 and 4S always felt like a solid chunk of glass.  The iPhone 5 with that extra lightness feels like a solid chunk of Oak or something of that density.  That might be part of the "niceness"--its density feels more like something natural.

Apple keeps making them thinner.  But at some point they'll get to a thickness and not be able to go farther.  More for working with a human a hand than materials.  Something as thin as a knife blade would be uncomfortable if not a dangerous thinness.  And far before that it gets thin enough it would be difficult to pick up off a flat surface.  That may be the deciding factor for just how thin a future iPhone could be; a pickup-from-a-desk test.

And from holding mine, I'd say that's about 1/2 the thickness of the current iPhone 5.

(And flexible displays only would work if they can become rigid after unfolding.  Can you imagine typing on a display that bent and warped or folded with every touch?)