Demon's Souls is like those tough as nails NES games where you die a lot but you progressively learn the game and through trial and error make progress. It brings back that sense of desire in knowing what's further and further ahead, which is something that has been seemingly going over the heads of developers these days. There's hardly a sense of wonder in videogames now, IMO, and that's why Demon's Souls made such an impression. It's hard, yes. You die a lot, yes. But it brings back that reward to the player for actually sticking with it and trying. Isn't that kinda how, oh I dunno, videogames kinda started?
I understand that dying over and over maybe isn't fun to some people, I get that, but for me and growing up on NES platformers and finally experiencing that childlike wonder again on a current generation game (and here I was thinking it was just me getting old, thanks Atlus!) in a generation where it's barely any longer about reward through challenge and trial and error, in a generation where you can just set it on easy mode and enjoy the cinematics and do QTE sequences to beat the game. Demon's Souls revealed to me that some developers actually still understand that philosophy and they've found a way to bring this back to us while utilizing modern visuals, controls, and online functionality.
This is how 90% of the videogames in my youth were designed (sans online obviously) and it's awesome to see that return. While playing it, you don't think at all about the graphics but about how to approach each situation, how to plan ahead, what got you killed the last time and maybe what you can do to avoid it. This is why graphics aren't the most important thing to me, and why I want to see great game design like this above all else next gen. Demon's Souls came out and proved that you don't need QTE's to have exciting, memorable confrontations (but rather skill), it proved that you don't need to have voice chat to have an exciting, memorable online experience, and in a game that is "always online, always connected," you can still cut the internet off and play the game without penalty. Because, after all, why should you be penalized for playing a videogame offline?
Basically Demon's Souls (and Dark Souls!) is sort of this unassuming middle finger toward almost every single terrible practice in the industry today, and that is one of many reasons why it's just plain awesome.