This is my favorite game of all time. While it lay the foundation for future Souls games, it still did some things better than any of the later games. Here are some of those things
The structure of the world and levels. Unlike the later games which have nearly fully interconnected worlds, here there are 5 completely separate worlds that each is accessed from the hub location. This brings with it some advantages. Some people have already brought up how it allowed them to go all out and give each world a unique flavor and aesthetic, when they didn't have to worry about making one interconnected world feel wholly cohesive. But the biggest advantage to me is what this structure meant for the flow of the game. For most of the game, there'll be 5 different levels available to the player that they can tackle in any order. This actually makes the game MORE open to the player than the interconnected nature of Dark Souls. For me, as I really struggled with the difficulty of the game, it meant I could approach it by trial and error, poke and prod in each of the different levels available to me, until finally I was able to find a path of least resistance, that allowed me to progress. It actually made the game a bit similar to old megaman games. Often, I would find a weapon or shield or ring in one level that could hugely help me out in another level. None of the later Souls games have given the player so much freedom in choosing the order to go through them. This was uniquely possible in Demon's Souls, thanks to how its levels are structured in separate worlds
The level designs themselves are uniquely atmospheric in that they're not afraid to sometimes make the player feel absolutely miserable. As an example, the poison swamps of later games are a complete joke compared to vast swamp in 5-2. While it may sound strange, the levels being able to make the player feel miserable is actually a good thing, as it really helps ground the player in the dark and bleak atmosphere of the world.
While many of the bosses are not as challenging as in later games, every single one is unique in its own way. Whether it be arena layout, or a unique mechanical gimmick, each boss fight has something special that makes it memorable.
My final point is difficult to describe, and it may be one of those things that could only be experienced at the time, and which may be impossible for the remake to recreate. But the original game had a feeling of otherworldlyness to it. And a lot of it came from its imperfections. Why do the different NPCs have their own hard-to-place accents, when they're all supposed to be from the same Kingdom? I love the soundtrack because of how well it fits the game, but in many ways the compositions are lacking and unfinished. The instrumentation often feels amateurish. All of these strange imperfections combined to make me question the reality of the game. They made me feel as if I as well was being driven mad like the inhabitants of this world were driven mad by the presence of these horrible demons
I can't wait to see how this game will be received by new players. Even though I fear that some qualities of the original may be lost in a remake that seeks to rectify some of the imperfections of the original, I am so happy that its status as a high profile console launch game will be able to bring in a lot of new people to experience this masterpiece