When it comes to ARPGs, I believe the difference between hardcore and casual is more of a state of mind and severity of focus.
A hardcore player will dive into the items, skill trees, and stats to really understand the nuts and bolts of the game. They’ll test their theories using multiple classes and builds against the hardest difficulties of the game.
The more casual gamer will look up build guides, play through the story, may tackle some higher difficulties before burning out on the game.
The difference is in drive and a curiosity to dive deep into the game systems to get the most out of it.