Hardcore players min/max everything as a whole. Their characters, gear choice, rotation. They may or may not play a lot, some hardcores only have a few hours a night to play, if that. But they maximize their time and plan ahead to yield the best output for their input.

Casual players play by their whim, but when focused on a goal often exhibit bursts of a hardcore mindset. However as a whole they care less about things like maximizing their dps by .7 by adjusting their rotation and generally just prefer to experience the content.

Nothing is wrong with either playstyle however.