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).

An elaboration of the system was given by Andrew here:

Merciful: You only ever need one save file, and that only if you want to turn the computer off and go to sleep. You never need to restore to an earlier game.
Polite: You only need one save game, because if you do something fatally wrong, it's blatantly obvious and you'll know better than to save afterwards.
Tough: There are things you can do which you'll have to save before doing. But you'll think "Ah, I'd better save before I do this."
Nasty: There are things you can do which you'll have to save before doing. After you do one, you'll think "Oh, bugger, I should have saved before I did that."
Cruel: You think "I should have saved back in the third room. Now I'll have to start over."