Dark Souls could be just another 3rd person medieval RPG, with inspiring dark visuals and atmosphere. However, what sets it apart from any other games, and what has made it popular, can be summarized by the sentence "It is challenging to the limit". And truth be told, it is not so easy to make a game "challenging" without falling into the trap of becoming tedious or frustrating. There are plenty of fake difficulty games out there, based on cheap shots or cheating AI. As someone who started playing in the 8-bit era, I can attest that.

But that's precisely Dark Souls' beauty and where the programmers have proved their craft and dedication. Even though it is not 100% perfect (there are still some unfair sucker punches at some places), the overall sense of challenge and achievement is one of the best I have ever enjoyed in a videogame. Every time you die, you will realize it was completely your fault, not some cheap trick, a need for superhuman reflexes or button mashing skills. You could have survived with the correct tactics, but maybe you kept swinging your weapon and did not pay attention to your stamina gauge. Or you did not properly block and let your guard down. Or you tried to hurry in a frontal assault on your foe, not caring about his attack routines and never retreating. But do not despair, since demise is not a mark of failure in Dark Souls, but an integral part of the game and the experience. It is just an opportunity to learn a new lesson, lick your wounds, and try again until you make it.

TL;DR: The Dark Souls saga is like a fanatical personal trainer at the gym. Prepare to be tested, but when you look back from time to time, you will realize it's a hell of a ride, completely worth it. Casuals begone...