DS3 introduced weapon arts and mana-based casting. While it took the community a few years to understand the usefulness of weapon arts, both it and magic matured into their final form in Elden Ring. Now good ol fashioned R1 duels are fairly rare in ER. Ashes of War and spam casting crazy big spells dominate in PvP. In PvE you could vaporize stuff in DS3 with magic and weapon arts to some degree too, but in Elden Ring you can sail through half the game never swinging your weapon.
Of all the things though, I'd say it was the concession to convenience and the addition of fast travel to the Souls series that really changed it from a really unique experience to something somewhat more commonplace. Some do say that there's no value to slogging through the same enemies over and over again and it's just a waste of the player's time; I'd argue though that 1) it created a really intimate connection to the game and added that degree of tension and paranoia that the other games lost. You really got to know areas and enemies like the back of your hand, and took in more environmental details just by sheer exposure to them over and over again. And 2) it totally affected level design. Which is what most people remember most about DS1, is how truly interconnected everything felt. You lost that with later games in the series. There are still good level designs, but nothing where it all felt tied together like DS1. Less of a sense that someone truly labored to achieve that effect. DS1 felt like the world got built first and the game came after. Later entries in the series had the opposite vibe.