RPGs: difficulty and headroom

This may interest you: http://nethack4.org/blog/strategy-headroom.html

The article defines "headroom" as  follows: if the choices a player can make in a particular situation have a wrong answer, that is a sign of low headroom, especially if a different situation would have a different right answer. 

Has some applicability to RPGs (stat roll = low headroom, point buy = high headroom) as well as general difficulty level (5E in general is very high headroom, but so is Master of Magic).