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/multi, 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 ceratin events/lore etc that must be done on hardcore?