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.
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?"
Dungeon World is a narrative game, at its core, that distinguishes itself from D&D in the way it tells stories. The innovations are in the core philosophies and mechanics. Let me address each of your points in turn:
Moves as Powers
Moves are NOT just powers. Many are closer to D&D's feats. Others have no mechanical effect at all. Some simply tell you that your character can do a thing, without giving any rules for it. The real key to class moves is that they provide distinct flavour to the characters, as well as story-telling hooks. Take a look at the paladin's Quest, or the fighter's Signature Weapon, and notice that they're more about interesting character details than they are mechanical effects.
Love this. A level one character and a level 10 character can coexist without any real issues, and being level 10 doesn't mean that early monsters are no longer a threat. DW's implementation of this isn't anything particularly special, but it works really well in the context of the rest of the game. Because stats don't increase by that much over the course of a campaign, you still end up with interesting results for die rolls (see Interesting Failure, below).
I also love this, but it's a tricky beast that takes a lot of getting used to. The GM should be directing the action and the spotlight such that every player gets a chance to be involved. But note that there are certain situations where it makes perfect sense to focus on one character or another. Combat scenes in DW should follow the flow of the action, not be restricted to a turn-by-turn basis.
Of course there's flanking. But it's handled narratively, not mechanically. See, for example, the thief, who gets to use Backstab on a surprised enemy. One of the ways to accomplish such surprise is for the thief to have positioned himself behind the enemy while another character distracts him. Or perhaps a bard chooses to taunt and distract an enemy, and meanwhile the fighter attacks the enemy from behind. Remember that if you attack something that isn't fighting back, you don't make the Hack & Slash move, you simply do damage (or whatever else you're trying to do).
Overall, the players have infinite freedom in what they can try during a fight. The key is to respect the fiction and pay attention to the situation. If there's interesting terrain, tactics are involved. If the enemy has only one weak point, tactics are involved. If there are many enemies, tactics are involved.
XP for Misses
Not earth-shattering, no, but this mechanic encourages players to try things they might not normally do, which is no small feat. Rewarding players for failure makes the players more invested, and even excited about failing. More on this in a second.
There are plenty of the innovations that you've missed entirely in your analysis. These tend to appear in the core of the rules, not necessarily readily apparent. They're apparent in the philosophy of the rules, if not the mechanics themselves. And many of them are in the GM-facing rules, not the player-facing rules.
Failure in DW is interesting. Note that all moves involving dice rolls have only three possible results: success, success with complication, or failure.
when you fail, the GM has every right (and in fact, is mandated by the rules) to drop a hard move on you. A failure is never simply "nothing happens." Instead, a failure leads to a change in the situation: you take harm (and not necessarily just HP damage), you get separated from your friends, you lose your stuff, you encounter some portent of future badness, etc. This is the real key, here, that keeps the game moving and keeps the stories interesting.
On that note, the most common result is success with complications, which is also interesting. Take a look at Defy Danger, probably the most commonly used move in the game, in which most successes come with a worse outcome, a hard bargain, or an ugly choice. Plain old success (e.g. you dodge out of the way of the falling boulder) isn't as interesting as complicated success (e.g. you dodge out of the way, but you either lose your weapon or your backpack in the landslide).
Mechanics that Encourage Storytelling
Take a look at the Spout Lore move. When a player request information of the GM, the GM is encouraged to ask the player how they know that information. The player gets to use that as an excuse to add information about their character (and the world) to the story. Or how about the Bard's A Port in the Storm? It mandates the GM tell you something new and interesting about the world every time you set foot in a place you've been before. Moves like these litter the rules.
Collaborative Character Creation
Players create characters during the first session, with each other and with the GM. The GM is expected (again, mandated by the rules) to ask questions, using the player's character choices to inform the setting. In the last game I ran, one player decided he wanted his Fighter to have scarred skin (as is one of the look choices on the Fighter's character sheet). I asked him how he got those scars, and he decided they were from fighting in a war. I used that idea to decide that the world was one that had only recently found peace, with tensions still running high while people slowly reconstruct civilization. Had the player suggested that he got the scars fighting as a gladiator, for example, that would have let to an entirely different setting decision. And that was just one question out of dozens that I asked the players.
And then there are bonds, which are statements of relationship between characters. This gets players thinking not only about themselves, but about how they relate to each other. The bonds themselves are vague enough to leave a lot of room for the players to come up with specifics, but defined enough to really cement the relationships.
There's probably a bunch more that I'm missing, but I've written enough, methinks, so I'll leave it at that. The other answers here are also really good.
It should be noted that most of these things aren't revolutionary in and of themselves. Each has been done before in other games, but what makes Dungeon World such a great game is how well all of it is put together, and how cleanly each mechanic is implemented.
Depending on your style of gameplay there are many possibilities that could happen immediately after failing that DD+DEX. With style of play I mean are you doing a rather slow paced, fundamental type of game, something radical, over-the-top or just some middle of the road not too unrealistic but at points more awesome than realistic?
Also it always helps just looking at the GM Move list. Actually, I'm doing that right now just looking at each and thinking what would make sense. Also remember, you can always make a break for a few minutes to think about fun stuff. Somone at the table always needs to pee/fill their drink/have a smoke anyway.
Just a few examples and the associated GM Moves:
use a location move: the mountain you're on is the mystical mountain that is know for making whole groups disappear. Use that location move: "Swallow them whole and suck them into your crystal belly".
turn their move back on them: you have avoided the avalanche, but you are now stuck to the cliffside. You are inside a pouch of air that maybe lasts a few hours. What do you do? (Disclaimer: I would probably only make them stuck like that if I know that they have a means to escape like a teleportation spell, maybe even by spending resources like a spell scroll)
offer an opportunity, with or without cost: you are being carried away by the avalanche! If you ditch your whole backpack right now, you might be able to pull yourself into that hollow treestump that is being carried around right next to you!
show a downside to their class, race or equipment, tell them the requirements and ask: your heavy plate armour is quickly pulling you down. You feel the weight of your shield being pulled at by the snow. The shield... The shield? The shield! If you ditch your heavy, gold-laden backpack now, you might be able to find your inner grace, get up on that shield and ride that avalanche to glory!
use up their resources: you are caught by the avalanche, quickly losing consciousness. Take 5 damage. (Dramatic Pause) Take 5 damage. (Dramatic Pause) Take 5 damage. (Dramatic Pause) You open your eyes. A shiver running down your spine. Candlelight. A steaming cup of tea to your side. The local inkeeper found you almost frozen, lying right in front of his mountain inn's door two weeks ago, when the avalanche swept you down. He and his beautiful granddaughter have been nursing you. You owe them your life, and surely a lot of expenses for medicine.
Also, always remember GM moves can also happen off-screen: