Monster Difficulty Should Increase Slightly Faster Than Characters’ Abilities
8. “Race you can’t win rule.” The game’s monster difficulty should increase slightly faster than the advancement of the [character], given average stats and default equipment, so as to force him to rely upon items and tactics.
The reasoning here is that if the player doesn’t have to rely on randomly-found stuff then [that stuff becomes] unimportant to play. However, if it’s required to have specific items to be successful then many games will be outright unwinnable. The balance between these two poles is what makes random dungeon generation difficult, but it’s also part of what makes random dungeon gameplay interesting….
Thank the gods for the Internet Archive Wayback Machine, else I could not have linked to the source blogpost.