Cruelty scale

A rating system devised by Andrew Plotkin to measure how difficult a particular game makes it for the player to progress past in-game obstacles:

Merciful: cannot get stuck.
Polite: can get stuck or die, but it's immediately obvious that you're stuck or dead.
Tough: can get stuck, but it's immediately obvious that you're about to do something irrevocable.
Nasty: can get stuck, but when you do something irrevocable, it's clear.
Cruel: can get stuck by doing something which isn't obviously irrevocable (even after the act).