I don't think it's flawless, hell after O&S the game suffers a pretty steep downturn in quality but it nails a bunch of things the following games don't, mainly exploration and having the feeling of it being a unique world. I can remember every single area in DS1 because they're all pretty drastically different. You have a dingy city, a poisonous swamp, a dusky forest, a crystal covered library, a trap filled fortress, etc. For DS2 the only thing that comes to mind is the same boring caves over and over, for DS3 the same kind of thing only outside of Irithyl it's run down shanty towns and cemeteries, for Bloodborne it's either the same black brick streets or niggerman central which suck because the enemies in those areas are annoying as fuck.
The second thing is DS1 not giving you teleporting until like 75% of the way through the game makes the world feel like a real place you have to pay attention to. In DS2 onward it feels like a bunch of levels with a few little optional paths because you can just warp wherever from the beginning. I think the open world in ER actually alleviates this somewhat since even if you're warping all over the place there's so much ground to cover it doesn't feel like you're just warping between hallways. I still wish they could have put in some crazy shortcut shenannigans in ER like DS1 had but I understand that's insanely hard to do in an open world.
