death/fear. 5e PCs seem too robust to my tastes. No more save or die, and no real fear of really bad creatures, like rot grubs, or level draining.
We had the same attitude, until we re-read the death saving throw rules more closely. You then realize any attack gives automatic death save failures. We lost a 9th-level paladin shockingly quickly because of AoEs and follow-up attacks.
We feel a bit more vulnerable now.
Damage at 0 Hit Points. If you take any damage while you have 0 hit points, you suffer a death saving throw failure. If the damage is from a critical hit, you suffer two failures instead. If the damage equals or exceeds your hit point maximum, you suffer instant death.