2. Invincible NPCs
When I hit someone with a sword made of Dragon Scales, forged in the heart of a lonely mountain, reenforced with all the enchantments and spells I have at my disposal and poisoned with the venom of a giant serpent I expect him to fall before me like Icarus, daring to confront me. What I don't expect if for them to drop, wait until I've started searching chests and the jump up and stab me in the head. Understanding that some players are required to survive in a game, in order to pass on instructions and so on is important, but Fallout fixed this by having NPCs simply drop journals from which the player can derive missions. Indestructible NPCs really remove from a gaming experience and contributes to scenarios in which the player flees from unkillable characters, low on ammo and stamina before being cut down and having to reload.