If we're using mechanics to represent the traditional tropes of fantasy narratives, then nothing is save or die. 'Hit Points' represent your plot immunity, and so an attack like a medusa's gaze would deal damage and blind you (representing you closing your eyes at the last minute). If it runs you out of hit points, instead you're petrified.
Honestly, when it comes to hit points, I, definitely, don't want them being used for things like Medusa's gaze.
I would prefer to see plot immunity come from a separate mechanic in the form of hero points
- receive a bonus to you d20 roll (must be declared before you roll)receive a bonus to you d20 roll (must be declared before you roll)
- re-roll a save (including a dying save) and take the better (maybe, with a hefty +10 bonus if you a roll a 10 or less on the reroll similar to True20 Conviction/M&M Hero Points). This would have to be announced after your roll, but before knowing if you succeeded or failed.
- automatically, shake off a status: dazed, fatigue, etc.
- automatically, stabilize from dying
- trigger a second wind
which would be in addition to other functions (e.g., take an additional action)
Finger of death, disintegration, word of death stuff, I have no problem with them doing hit point damage or having a two save process- an initial save to avoid and then go into death and dying save upon failure.
Poisons, I, definitely, have no problems doing hit points (as well as secondary effects). I would like to see them have an onset time and be continuous. I would, definitely, be looking at Poisoncraft (Blue Devil Games) and Sean Reynolds
variant rule: continuing damage for poison).
Then again, I would like to see hit points less abstract and something that does not scale or does so minimally. Though, I do like how characters start with more hit points in 4e.)