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.
No Scaling
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).
No Initiative
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.
Less Tactics
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.
Interesting Failure
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.
More
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.
Best Answer
There's a possible “next” there, but if you've come to this point you've already broken a cardinal rule and are naturally suffering the consequences. So the thing to really do is to back up, remember the GM's Agenda, and follow that agenda instead of following old D&D-conditioned DMing habits.
Specifically, with emphasis mine, your Agenda is expected by the game to be:
By creating a climactic battle and planning for it to take up the session, you're departing from Dungeon World's rules framework and entering uncharted territory where all the rest of the game's rules will start falling apart or having undesirable results when followed (such as the Barbarian's Smash! move).
(You can tinker with Dungeon World's rules, but it's surprisingly easy to get a really unsatisfying game by doing so without first having a deep understanding of how they all interlock. Accidentally not following them is an even better way to discover ways it can work unsatisfyingly.)
Play to find out what happens
Dungeon World demands improvisation. It doesn't say that and then leave you hanging though: every single rule in the GMing chapter is part of a framework for that improvisation, so that you can improvise without ever running the risk of having no idea of how to improvise next when its your turn to do so.
To do this though, DW demands that GM let go of planning. A GM following the game's rules is literally not allowed to plan ahead. They may create possible situations, but never outcomes; brainstorm ideas, but never force them to happen; brainstorm new and interesting ways to use GM moves, but never use them when unsuited to the growing fiction.
You are even allowed to prep a whole situation… but you're explicitly not allowed to use it as a script! You are instead instructed to exploit your prep and mine it, dismember it, steal from it the pieces that end up fitting the game that unfolds at the table.
Improvising means being reactive (like a player!)
A Dungeon World GM is almost entirely reactive. This takes some time to get used to, and often takes some unlearning of GMing habits from other games.
For example: many of the examples of how you might answer this question about D&D are against the GM's rules in DW:
Yeah, sure! Sometimes you might be better off with a few different baddies than one singular one. But sometimes the fiction demands a single Big Bad, and it's a rule: Begin and end with the fiction.
Not allowed! The only time the GM can do this is when a PC gets a 6− or when they give you a Golden Opportunity.
Also not allowed, for the same reason.
Indeed, this is unfun! It's also not allowed, because it violates the GM's Principles to Address the characters, not the players, Be a fan of the characters, Begin and end with the fiction, and Think dangerous.
“But no, really, what should I do when that happens?”
Suppose it's the climactic fight, you've just started the session, and the Barbarian up and decapitates them just like that. Let's suppose that you've followed the rules and only prepped the potential situation, not planned it out.
What you do next is describe the world and say how the head comes off and lands at their feet / is raised up in the Barbarian's fist / whatever suits the details established already. Do that, then ask the question “What do you do?”
Then the players will tell you how the session continues. Respond with GM moves if and when the rules call for them, notice PC moves triggering and follow them, and keep playing to find out what happens. Let your players surprise you…
But keep thinking dangerously. Keep thinking offscreen too. Just because the Big Bad is dead doesn't mean the session or the campaign is done quite yet; it definitely doesn't mean they're out of danger and excitement. Play it out, find out together what happens based on what they do next, and know that a missed roll or a Golden Opportunity still lets you make a GM move. The game is still on!
Yes, the players might say “Uh, we go home now” when asked what they do, but just like at every other moment in the game they need to explain how they do that, so you should respond with “okay, how?” A player saying what their goal is never enough for their PC to accomplish it! They're still in the game, and have to say what immediately-possible action they do next, not just what their next goal is. And as soon as they start saying how, you're back into the fiction, into the action, and back to the possibility of triggering moves.
But more likely than not their answer won't be “Uh, we go home now” anyway. More likely it will be something like searching for the Big Bad's treasure vault (is there one? play to find out!), burning the place down, attempting to recruit the newly-jobless minions, taking over the base, turning the head into a grotesque trophy, or any of a thousand different things players pumped by a major victory tend to want to do. Going home might be their answer, but it's not the only or even most likely answer. So ask!
TL;DR: When the big boss is killed instantly is not a trigger for any GM move, so Dungeon World doesn't let you do anything to fix or mitigate it. Don't try to.
A sudden and early end to a big fight is not a real problem for the game engine, and doesn't need solving. The game's normal rules will continue to make the rest of the session interesting — possibly in unexpected ways that are more interesting than a the big fight would have been! (Who knew the Big Bad had a planar portal in their private sanctum? These things can just kinda… happen… when player moves and GM moves interact.*) Assuming that a quick end to the Big Bad is a problem that needs to be avoided or solved is a habit carried over from other games that can only interfere with letting the Dungeon World game engine work properly.
When the engine is allowed to run properly, one of Dungeon World's greatest strengths shows up organically: every unexpected event leads to things that are more timely and interesting than what most GMs could have planned for. Instead of panicking and assuming the session is ruined, just calmly keep following your Agenda and Principles, and (when allowed) keep making GM moves. The game will keep going, and what comes after the Big End Fight might even end up more important than the Big Bad.
* As an example, I once invented a bottomless pit caged by a carved stone screens, on the spot as the PCs entered a room. A bunch of player moves and GM moves later, the druid was thrown at it, turned into an elephant mid-air, and awoke a primordial being dwelling in the elemental darkness the pit lead to. (All of those stages involving player and GM moves, each one's consequences improvised on the spot, per the rules.) This ended up introducing what turned out to be in hindsight the main storyline of that entire game, and none of it was planned by me. They ended the game by raising a new demigod in the Forgotten Realms, which was much more epic than anything I would have planned for in a different game system. Bringing it back to the point: it was way more interesting than the fight that caused the druid to been thrown in the first place.