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.
A lot of the issues I have with DS 1 just comes down to the placement of bonfires and NPC's. Bed of Chaos is the most egregious example but there's more where if you fail you have to backtrack 5 minutes to get back to where you were. Or, if you need to get to a specific NPC like a blacksmith that's another 5 minutes of running to get there.
The other issue I have is that once you get fast travel you cannot travel to just any bonfire and that just creates more needless time wasting. I'm not saying fast travel needed to be available from the get go. But once you get it there is no reason why you shouldn't be allowed to travel to any bonfire you have visited just like every other subsequent game has done. It's for that reason whenever I go through Blighttown to fight the spider I don't rest at a bonfire and just homeward bone out of there the instant I beat the spider.
Then there's other niggling things: getting boss weapons didn't need to be made so very cryptic. Then there's some boss fights that are just bad. Capra Demon isn't difficult at all it's JUST THAT FUCKING TREE THAT BLOCKS YOUR LINE OF SIGHT. That tree didn't need to be there. Because of that tree and the extremely small arena it forces you to play a very specific way to just kill the Capra Demon ASAP. Oh, and if you failed at the fight because of that fucking tree or the hit-detection just fucked you over when killing one of those dogs and you get stun-locked and the Capra Demon one shots you, guess what? ANOTHER 5 MINUTES OF RUNNING BACK TO THAT SPOT.
Centipede Demon: terrible fight. You have to play a very specific way and just stay on this narrow platform waiting for it to come at you. Not a hard fight at all once you know what to do but it's badly designed.
Bed of Chaos needs no introduction. The main problem I have with this fight is the 5 minutes it takes to get back to this boss every single time you fail. Just put a bonfire next to the boss and the Remastered version could have easily done that.
Gravelord Nito: again, not a hard fight once you know what to do. But, staying in a little spot and just hugging the boss and tanking aoe or equipping a holy weapon to get rid of the basic bitch skeletons was just poorly thought out.
I don't know what to think of the Four Kings but it suffers from the same problem where if you fail you have to backtrack 5+ minutes.
Oh and stun-locking is bullshit. I was glad to see that gone in the later games.