Why has Dark Souls achieved mainstream success and has not remained merely a cult favorite? I'd like to argue that a major factor behind Dark Souls' success is the disconnect between its perceived difficulty and its actual difficulty. Dark Souls presents itself as an impossible challenge to the player outwardly, but inwardly, the game is subtly designed in many ways to help the player achieve the impossible.
(Note: this article includes a few spoilers.)
The publisher of Dark Souls actively sought to brand the game as a difficult game from day one. Just look at the name of the game's official website: PrepareToDie.com. Marketing the game as being extremely difficult increases the game's perceived difficulty without changing the actual difficulty at all.
2. It lets the players make their own rules
After stacking the odds against the player (with a huge, hostile world), the game starts stacking the odds back in the player's favor. First things first -- Dark Souls doesn't force the player to play the game in any particular way. Want to play a heavily armored knight, a light-on-her-feet warrior, a mage, a priest, or all of the above? Sure thing. The game lets you use the style of hero that you feel most comfortable with. Although the player chooses one of several set classes at the beginning of the game, class selection only determines the player's starting stats and equipment; where you go from there is entirely up to you.