This seems, to my mind, a remarkably concise and remarkably useful definition of difficulty. It takes the point of Bad Trap Syndrome (another classic post influential on my thought) and generalizes its conclusion. A fight without this sort of difficulty is, much like Robbins' bad traps, just a resource tax; a mechanical transaction of HP and spells for XP and treasure with an exchange rate depending on luck, with little impact on the players themselves.Contrary to what years of WotC-D&D have told you, a “difficult fight” is not simply one where the characters’ resources are stretched or used up, it is one where the players cannot rely on their usual tactics and still win, regardless of how their characters end the scenario. In other words, even if the characters are beaten, bruised, and bloody at the end of the scenario, if they win without the players having to stretch their imaginations to figure out some new tactic beyond what they conventionally use, the scenario is not really difficult.Because the game is about the players’ experience; the characters act as a conduit to that experience.
We had just such a series of difficult fights two sessions ago, with the ankhegs of Opportunity. Initially the party considered engaging them directly, but decided that this would be suicidal and so spent a bit of time thinking. Matt was eventually reminded that his character was capable of building traps "strong enough to catch a wyvern or an elephant", and so deadfall log traps were designed. The ankhegs did not sit idle while such traps were being built in front of their lairs, however, and Bhoskar the Dwarf was devoured while the first was being constructed. Further thought was given to the matter, and it was concluded that perhaps flaming oil should be used to drive the creatures away from the trapping project. This was tried with more success; Jarol the Cleric was nearly dragged away, but escaped the mandibles of doom and managed to light the oil, at which point the creature fled. After further deliberation, the players finally had an Old School Epiphany - when in doubt, bring pack animals to reduce the likelihood that you will be the one getting eaten. So they went and bought a herd of cows, which they used as bait while setting the traps. No further casualties were had, and with this method the ankhegs were finally exterminated and the peasants liberated from their hungry jaws.
And I've finally got a player willing to do write-ups for bonus XP! His (highly propagandized) accounts will be going up on the Obsidian Portal.