I've run through the game on Normal, Nightmare, and Hell (and parts of Inferno) on non-hardcore and feel like I've seen the same or very similar dungeon layouts throughout regardless of character, level, solo/multiplayer, etc.
Conversely, on hardcore I've never seen the same dungeon twice. I've also seen sections of dungeon (appearance-wise, etc.) that I never saw on non-hardcore.
Are there discrete sections of dungeon that only spawn if you are in hardcore mode? And if so, are there certain events, lore, etc. that must be done on hardcore?