Introduction
A miss means that the character's action is unsuccessful or carries major consequences. Unless the move tells you what to do, all moves work the same on a miss—the GM takes action, doing something dangerous to the characters.
Chapter 12: The GM
You also make a move when the players give you a golden opportunity. A golden opportunity is any time they ignore a threat or when they fail a roll (6-).
When they give you a golden opportunity, you can make your move just as hard as you like. A hard move is one that is irrevocable and immediate. The players immediately feel the consequences of the move and have to deal with them. Dealing damage is a hard move, since the damage is immediately applied.
On a miss, the GM makes a move. In particular, the GM is empowered to make a hard move, which includes just dealing damage! Of course, the response should be guided by the GM's principles, including make a move that follows (in the fiction). If you miss Hack and Slashing an orc, then it would follow for you to get hit by the orc. It wouldn't necessarily follow that a cave-in occurs, unless there's been some buildup in the fiction that a cave-in is about to occur.
First off, all of edgerunner's answers are great. But I wanted to add some Dungeon World specifics:
Check p.19 and you'll see that 6- isn't "failure" - it's "trouble". The GM will say what happens and the player will mark XP. You are attaching non-DW simulationist ideas to DW mechanics by your supposition that 6- means "failure."
These principles can apply in all sorts of games, and have been used by GMs for years. If the PCs have to climb a fence, they're just going to keep trying until they succeed, right? So even in traditional games, many GMs will read "failed" rolls as a lack of some quality - not fast enough, not quietly enough, not without hurting themselves, etc., instead of just keeping them on the wrong side of the fence.
This is because failure is boring and stops moving the story forward. So you are correct, there is no plain-old failure in DW. It's not in the GM's agenda to make the PCs fail. There is no move for failure.
So the problem isn't that edgerunner's ideas are non-optimal, it's that your concept of what 6- means is wrong and that static failure doesn't exist in Dungeon World.
Expanding on 6-
From the text:
Generally when the players are just looking at you to find out what happens you make a soft move, otherwise you make a hard move.
Somewhere in Apocalypse World itself it says about hard moves:
make as hard and direct a move as you like
Early PbtA games like DW assumed you understood Apocalypse World. And this phrase is often tacitly implied in PbtA games even today.
6- means trouble as I said. The GM is free, on 6-, to make a move as hard as they like. That doesn't mean as hard as you can think of.
AW says:
It’s not the meaner the better, although mean is often good. Best is: make it irrevocable.
So while a 7-9 should substantially give the character what they wanted (they accomplish their intent even if their action created complication), on 6- you are free to deny the intent (the action still has to have consequences beyond "no" though) and in addition make a move as hard and direct and irrevocable as you like.
Climbing a mountain a soft move is "The boulders above you on the rock face begin to wobble as the grappling hook you've tossed up there sets itself. What do you do?"
A harder move is "The boulders have tumbled off the edge of the ledge and after hanging nearly motionless for a tiny instant above you, are now plummeting towards you, gaining speed every moment. What do you do?"
A really hard move is "The boulders are yanked free by your grappling hook and come smashing into you, tearing you from your narrow perch and scattering the contents of your pack into the yawning emptiness beneath. What do you do?"
Best Answer
So, after this question pointed me at that hack, I convinced the group I GM for to try out a campaign with Descriptive Damage. I'll describe our experiences so far, though they should be taken with three caveats: (1) all of us are still comparatively new to Dungeon World, so things might play out differently with a more experienced group (or at least a more experienced GM), (2) we've only played two sessions so far, and (3) the hack left many details up in the air, and different decisions on our part might have led to a very different game.
Preliminaries: Our Rule Selections and Interpretations
We played with Resist Harm triggered by any injury to a PC. Damage to monsters was as in regular Dungeon World.
We played with standard HP totals and HP regeneration at a rate of half the character's class's bonus HP per level-up. Magical healing was as in vanilla DW. I did not use HP handouts to incentivize "doing fictionally and thematically relevant things".
We did not cap HP expenditures.
A character protected from an attack by armor took +1 to Resist Harm. A character impervious to an attack because of armor took +2 to Resist Harm. (We also considered abstracting away armor ["…if you have lots of hit points, it's because you're badass and probably wear heavy armour"], but the first encounter was with a razor boar, and it helped to have the monster's fiction backed up by some mechanical consequences.)
Spell Defense was treated as giving +1 to Resist Harm.
We didn't use monster "deadly" ratings. Injuries were deadly or not as adjudicated from the fiction.
We couldn't figure out whether Last Breath was supposed to be in-play or not, so, as a death-adverse group, we kept it in. However, it hasn't come up yet.
Aspects that Played Well
Specifics for Injuries
Playing without Descriptive Damage, our group often has good fictional detail on the how of an injury, but fewer specifics the what; with Descriptive Damage, we got both, even though none of the injuries I threatened were particularly exotic. For instance, our cleric, instead of just being beaten and bruised, has a nasty gash in the shoulder of her off arm that got infected when she fell into some filth, plus she's dealing with some lingering effects from a concussion. Our druid got stomped on by a gargoyle while in snake form, and now she has some broken ribs and a crushed lung. Our wizard broke two fingers when another gargoyle wrestled a book from his hands. Etc.
Threat Differentiation
Since character's injuries were specific, the fiction picked up situations where certain monsters could exploit or exacerbate those injuries, making those threats play differently than indiscriminate ones. For instance, after slicing her shoulder, our cleric was still willing to get in a tangle with a roper since it was just trying to eat her. But she was a lot more skittish around a pack of gargoyles that could see and target the rent armor on her left side and her mostly useless left arm.
Balance and Player Spending Strategies
In the hack comments, Paul writes:
This was our experience. Most of the time players spent 3 HP (for a 1/12 chance of missing) but they were willing to put up more if they felt that avoiding the injury would let them wrap up a battle. So, roughly speaking, injuries under Descriptive Damage corresponded in magnitude to damage from a solitary monster with a damage die around 1d8, though, because of player agency, with much less variance. At least against solitary monsters, the game balance didn't feel like it shifted much.
Aspects that Played Poorly
Confusion about "React Immediately"
One of the choices for Resist Damage is:
In the first session I read "react immediately" as a bonus to give a character: little or no in-story time would pass between their injury and their next action. This played a little oddly.
For instance, one of the times our cleric went for the razor boar with her mace, she missed, the boar inflicted an injury, and she rolled an 8 and chose immediate reaction, which she used to Hack and Slash the boar anyway while she was being gored. Characterization-wise, that was cool, but it made the H&S outcome more like a 7–9. And if she had rolled 9 or less on the second attack, I would have had to come up with another way for the boar to deal damage, which could have triggered an immediate reaction….
Reviewing for the second session, I came away with a different impression based on the paragraphs before the move. Instead, I read "react immediately" as a character's usual ability to keep taking actions through a scene and the absence of "react immediately" ala the Frodo example. That led to less dissonance between the fiction and the mechanics.
HP Mechanics that Crossed the In-Fiction/Out-of-Fiction Line
We had a lot of difficulty agreeing on whether or not HP corresponds to anything in the fiction, mostly because either choice led to immersion problems.
HP as "Plot Immunity / Mistake Potential", Paul's likely intent, while supported by the hack itself, interacted jarringly with other DW rules, particularly ones about healing: HP outside the fiction but HP management in the fiction makes for disconnects between player and character motivation. Clerical healing, as mentioned in the hack comments, is the trickiest case, because it's unclear story-wise what the cleric is doing or why. We also didn't know quite what to do with potions, and our halfling druid had to invent a justification for singing her healing song in the middle of the night out in the woods when the party was trying to avoid attention.
On the flip side, attempts to equate HP with "Defensive Combat Skills / Stamina / Luck" presented problems with the hack rules not being designed for such equivalences. Skills aren't really something that threats of injury consume; stamina is something that should also be depleted by other actions and replenished by rest; luck normally applies to more than just injuries. One might be able to swing an explanation that uses a kind of divine favor or luck specifically for self preservation, but I feel like it makes more sense to rethink the DW HP rules to fit the hack than vice versa.
Deaths by a Thousand Cuts
While differentiating monsters by threatening different injuries works well for the heavy hitters, monsters that wear PCs down slowly don't seem to work well under Descriptive Damage. Or, at least, I don't seem to be clever enough to describe injuries that players want to spend just 1 or 2 HP on. As a consequence, those monsters become a lot more dangerous.
I think part of the problem is that I always gave injuries with all three kinds of effects: immediate, temporary, and lasting. If I only threatened one, say, then hopefully players' spending would be motivated by their chances to reach the 7–9 category instead of the 10+ category.
Aspects that Played Differently
Duration of Consequences
Under Descriptive Damage, injuries are about equally frequent as in vanilla DW, but their consequences are a lot longer lasting, especially because Recover from Trauma can have some steep time costs and still not provide a full recovery. In our game the party has just barely uncovered one front, and already every character has at least a few months to wait if they want to heal naturally; it takes some believing that they're not packing up and heading home. I may rule that a potion erases an entire injury, or else we'll end up with a much grittier feel than we normally like to play with.