another issue is the size of one's hands. i can span an octive and 2 notes with one hand on a piano, and sometimes find that keys right next to WASD feel somewhat cramped (especially E, Z, and X, as other fingers tend to get in the way or block the view). with my middle 3 fingers on AWD and my hand in a relaxed postion, my thumb is 1/2" below the spacebar, and my pinkie is 1/2" to the left of left shift. making those the two easiest keys to hit. ironic, or perhaps subliminal, as left shift is sprint, and spacebar is use (currently), and in the game you mostly interact with stuff and run from encounters! <g>.
Ah, ergonomics and interface design, there are no easy answers. I actually started out developing operating environment systems, before i started making games. So i've always been interested in (and confronted by the challenges of) good interface design.