Let's see why Diablo II is still the benchmark,
- Well done art direction that's hard to dislike. No oversized heads, big eyes, white teeth, but mature gothic horror. Back in the day everyone played Diablo, including your sister or grandma.
- Skills that feel good and crisp even after 20 years. Look at Zeal. It has no particle effects of other VFX. Just really well made foley effects and animation. It had a very satisfying CHUNK CHUNK CHUNK sound like you were really hacking into flesh and the monsters went ER AGH URGH with each hit, then a loud satisfying death scream when they die. Today, devs are obsessed with making skills *look* good, and that's all, none is trying to make them truly *feel* good.
- Good class identity and balance. None of them really felt samey or bland.
- The inventory is pretty to look at. We spent hours just sorting out stuff. Item icons are vivid and realistic. The inventory is like a minigame in itself. Celtic motifs on UI banners and other elements are never boring to see.
- Nearly every monster had a unique mechanic linked to it. Some monsters explode, the others have a "charging" type of attack, some multiply when you kill them, etc. We had fun without dodge rolls and Soulslike bosses, can you imagine.
- Lighting in interiors was really good. You always wondered what's in the next room. Color palette was grounded and supported this feeling of a doomed world with an uncertain future. It felt like you would get raped by monsters if you leave town. Can't do that if your game is full with wild, zany pink colors.
- Diablo revolutionised system design. Skill trees, the red and blue mana "blobs", item parameters. For better or worse, but it did revolutionise it. So no one is going to play the same thing over and over again. They need to come up with something new, but not in a politically deranged, SJW type of way.
To summarize, it's about creating a feeling. When you do that, everything and anything in your game will feel iconic. But there are so, so many things that go into that. Nowadays devs don't put as much thought into it as they used to. I'm probably forgetting some myself.