To set the stage for the rest of my Demon's Souls discussion, I'm going to take a guess at the experience its designers wanted me to have and the lens through which design concepts were created. I have no privileged information -- I may be wrong -- but it will help describe my experience and illustrate a hypothetical example of how strict focus on a game experience can drive interesting gameplay.
If I had to, I would guess that the design of Demon's Souls was born from an intense focus to rationalize death in an internally consistent way within a game world. Why would we respawn from death? Would we be trapped in limbo, cursed to repeat the same pointless exercise? Is there any escape? Are other people trapped as well? Why? How would a world look haunted by so many with no ability to pass on? And if death is treated with realism, how are other game oddities explained, such the affect of attacks on NPCs, and the strange knowledge players have in a foreign game world?
These are the interesting questions I would guess the developers asked, and Demon's Souls was Atlas' best answer to these questions. More to come!