Out of all the mechanics, environments, storytelling, etc, what features did Demon's Souls do the best job of implementing over the other two souls games?
Personally, I haven't actually played it, so I can't quite say. --- Currently Playin': Tales of Graces F, Persona 3 FES, Persona 4, Okami HD. Anticipating: Xillia 2, Watch Dogs, Pokemon, Smash Bros, Mario Kart.
People tend to agree Demon's Souls is dripping with atmosphere. I throw my hat into the ring of best NPC's. Yurt, Urbain, Ostrava, Freke...Yuria. --- How could a Demon ever stand up to a Bonafide Knight? Official Gyrm Greataxe of Dark Souls II
Best atmosphere and level design by far; the places all really felt like locations and environments. At almost no point did I think, "This sure just feels like a pointless corridor." A feeling I got sometimes in DkS, and 99% of the time in DkS2.
I also have to submit a vote for best NPCs and their accompanying voice actors. Hearing Yurt in that cage, or Astraea requesting that you leave, are lines I will never forget.
The amors are also the best designed, albeit with their near futility in PvP.