An example of save or die done poorly:
DM: “As you descend the stairs, waiting for you at the bottom is a woman with a wretched face and snakes for hair. Make a Con save!”
PLAYER: “Wait, I close my eyes!”
DM: “Too late, by the time you’ve seen her you have to save.”
PLAYER: “Well I already had them closed!”
DM:“While you were on the stairs? Your character would have no way of knowing there was a Medusa at the bottom. Besides, you never said you had them closed.”
PLAYER: “I hate this campaign.”
An example of save or die done well:
DM: “As you traverse deeper and deeper into the dungeon, you pass statue after statue, each of people in adventuring gear much like your own, each frozen with a look of surprise on its face. Not a one has their weapons drawn.”
PLAYER: “Sounds like the Medusa who lairs here petrified them in a single glance. Good thing the villagers warned us she’d be here. I’m still navigating with that mirror I bought, by the way!”