One of the things FromSoftware is incredibly good at is creating an atmosphere and feeling around their game worlds. Nowhere is this more obvious in their enemy design. Dark Souls isn't really that hard once you overcome one simple, critical weakness that the majority of new players succumb to:
Fear.
Dark Souls is a master of the intimidation factor. Bosses, especially, but even regular enemies are gruesome and relentless and the potential loss of souls makes them all the more frightening. The reason Souls veterans are so much better able to assault new content is because they've realized that they can't fear the enemy. They've realized that their 100% block shield actually blocks damage even from huge scary shit, even when it seemingly shouldn't. They realize that Dark Souls is so much easier if you just walk past all the enemies rather than fighting them over and over. And, most importantly, they've realized that dying really isn't a big deal in DS. Once you conquer your ignorance and fear, DS isn't that much harder than your average ARPG.
There's also the environments and locations. When I first stumbled into Blighttown, I was scared shitless of falling off to my death, being poisoned by toxic needles and running out of purple moss, or getting swamped by the enemies.
And there's also that quickening of your heartbeat when you get invaded at critical moments.
But in the end, it's just timed blocking and dodging. Once you master parrying in Dark Souls 1, the game feels less overwhelming.