I personally think of it by how much each party member's responsible for the clear.

Hardcore, starting with Extremes from ShB onwards, generally expects everyone to do the dance right. Mistakes made by any individual usually blatantly penalizes the entire party. There's much less room to make up for the mistakes, and that naturally makes it a time sink, so the investment becomes more necessary.

Midcore, which I see as things like Bozja's CEs, CLL, and DR normal, can put just as much strain on the individual and punishes them hard, but typically only slows down the run, not kill it entirely with a body check or hard enrage. A few good players can drag the rest of the team across the finish line if it comes down to it.

In that sense I think normal content can sit around that Midcore line when they're still new, but they quickly become outgeared within the same patch, to the point mechanics become regularly skipped and some bosses may not even get to do much more than their tutorial phase. Chaotic falls under Hardcore for me.