What really made Demon's Souls lose it's appeal to me is that given the game's difficult nature, you're expected to know every level inside and out. Every nook and cranny, every spot where an enemy will pop out, everything. I get the most out of a game when I'm playing it for the first time, when I don't know what the next level will have. When you can play an entire level using your mind's eye, (and I know you can) then there's something wrong.
Don't get me wrong, I love Demon's Souls and have played it to death, but after awhile, you just tend to lose that euphoric sense of discovery that games like this have. Ever get 'lost' in a video game? That state of zen where you forget that there's a controller in your hands, and this isn't a game, not anymore; it's an experience. That's what I'm talking about.