I’ll compare some of my main observations.

Environment: Dark souls 3 has incredibly cluttered environments in a good way. Every room has stuff in it, corpses and furniture and little details that make it feel alive, claustrophobic and horrifying. Compare that to ds2. So many rooms are just empty, empty boxes with nothing in them. This significantly changes the atmosphere of the game and once you notice ds2 is just empty rooms you can’t unnotice it.

Combat: Actually ds2 was pretty good except for hit boxes. But ds3 borrowed most of the good changes from ds2 anyway and dropped the (imo) bad ones like healing stones. The bad part of ds2 combat was definitely soul memory, an absolutely horrible mechanic for longevity of pvp in non designated pvp zones.

Vibe: already talked about this a bit in environment, but ds2 feels like a Skyrim adventure. It isn’t scary. Hollows are just zombies and there’s never any really scary moments or eldritch creatures, it feels sanitized almost. Ds3 had a lot more eldritch horror and that helped complete the vibe.

Bosses: so many of the bosses in ds2 are just bad or cut and paste. So many. Both rat fights are dumb, cut and paste dragon riders, so many fights are just “big knight with big weapon”. All the best fights are either the end of the game or the dlc. Contrast that to ds3 where the bosses are challenging but fair and have much more polished themes, more varied movesets, more lore, and are just in general more fun. The only truly bad ds3 boss fights imo are the one dragon that gets one shotted and wolnir.

Enemies: the game design philosophy of ds2 is just “put tons of enemies with huge lock on ranges everywhere”. Much less polished than ds3 and it feels artificially difficult. This was somewhat fixed in sotfs but not enough.

These are just some of the reasons, I could probably list a few more.