One thumb use A mobile device I want to operate with one thumb: holding it in one hand and using the thumb for everything. Now I don't want to give unnecessary details about the lengths of my body parts in this forum, but let me just say that I can comfortably move my thumb around an area of roughly 1.6"x1.6" (or 4x4cm). On a touchscreen device, this would mean a 2.2" screen that'll always be covered by my thumb. (Go and watch people who use touchscreen phones, no matter which brand: They'll always hold it in one hand and tap on it with the other hand. That's a compromise I'll take for a portable device such as the tablet, but not for a phone that's supposed to be made for mobile use.)
Tactile feedback I need to be able to operate the device without looking at it. It's a mobile device, so I do things with it while I walk, while I talk to somebody, etc. With hardware buttons, I'm always certain what to do: One to the left, one down, press. Center, press. Quick look at the display, one up, press. You cannot do this with a touch screen.