What I miss most from Demon's Souls is how different every playthrough feels. After Phalanx, what path you choose depends entirely on what you feel like doing or what build you want to make, and a lot of mechanics facilitate this. Weapon upgrade materials differ based on your weapon and are found in different areas, your healing is done through consumables and thus they don't have to scatter flask upgrades in random locations you may miss or not get to until later, and each Archstone is self contained, allowing you to to go elsewhere to prepare for the more difficult sections or challenge yourself and get through one Archstone in a single go.
What I miss most from Dark Souls is the interconnected map. The reuse of old areas as bridges between new ones creates a tangible sense of growth and cohesion, and holding fast travel behind progression forces you to familiarize yourself with the environment. Even with fast travel, it's limited to certain "key" bonfires, and still could encourage deeper familiarization (if the second half didn't drop the ball in that regard).
What I miss most from Dark Souls II would have to be the build choice and variety. Magic is good, melee is good, bows and even crossbows are pretty effective, along with a greater lean towards experimental design. I really appreciate the idea of infusable special weapons, to suit a certain playstyle even more, and being able to buff most weapons on top of that makes infused weapons much stronger than standard ones (though I think that may have been a bit much). After all that, it has great visual variety as well, with a wide colour pallet and unique armour designs not really seen elsewhere in the series.
What I miss most from Bloodborne is the player moveset. Dashes while locked on, every weapon getting unique functionality, the way backstabs work, I find it's the most solid out of all the games. Parries working off of an attack rather than matching invisible frames with the enemy's invisible frames is another grrat change, as well as it making non human enemies susceptible to it. Pretty much all that I like most about the player's kit could be translated to better fit a different, slower style of game.
What I miss most from Dark Souls III is the consistency in its quality. It's low points aren't as low, and it has many of the best bosses and areas in the series. I've played through Dark Souls III the most, and I think the biggest reason is that it doesn't have as many quit moments or major fundamental flaws as the other games. It's not just consistent, it's consistently great.
What I miss most from Elden Ring is the presentation. The Lands Between is downright gorgeous, even the parts covered in rot. It's the only game with ambient and combat tracks, which help set the tone. The other games aside from Bloodborne didn't quite nail the look they were going for, aside from a handful of areas, but Elden Ring's open world, legacy dungeons, and large underground areas all look wallpaper ready.