(First, I'm not a lawyer, this isn't legal advice, retain a lawyer if you have any doubts.)
Referencing things has nothing to do with copyright law
Let's get one thing out of the way: Dungeon World's references to bits of pop culture and literature and stuff is in no way a violation of copyright, trademark, or patents. Trademark and patent law don't even remotely apply, and copyright prevents copying and transforming a copy, not plain referencing. Those are the three prongs of IP law, the entirely of what creators have to worry about, and none are relevant to referencing pop culture. This will probably make the rest of this stuff make more sense.
You can make stuff for Dungeon World. Please do!
Dungeon World is licensed under the Creative Commons Attribution 3.0 Unported License. That means you can make anything you want using even its text, and you can sell it so long as you give attribution to Sage and Adam as the creators of what you're using. You could even just copy the, say, Druid class wholesale, call it "The Shaman" without changing anything else, stick an attribution line at the bottom, and put it up on RPGNow for $5 and that's legal. See "How do I properly attribute material offered under a Creative Commons license?" for details on how to correctly attribute Dungeon World as the basis of your work, if you end up using any of its text (like, say, borrowing a Wizard move whole).
But for just a class, you don't even need that. Publishing a class that's compatible requires no use of copyrighted material from the original, so you don't even need the CC license in that case. You just publish.'
Even saying "This is compatible with Dungeon World" is legal: that's called "nominative use", and the use of a trademark to indicate compatibility is a protected right that you have. TSR went to court over this ages ago, and lost hard, so it's even a fact that's been tested in the RPG world. Kenzer Co openly publishes material for D&D without using the OGL, because they aren't copying any text and it's legal to say "This works with Dungeons & Dragons" without getting WotC's permission.
Besides which, Sage and Adam encourage people to make stuff for Dungeon World, an have provided a "For Use With Dungeon World" kit that you can (optionally) use if you want a recognisable shared graphic.
Lots of people have already created new classes for use with Dungeon World and are selling them just fine. Not only are you in the legal clear several ways from Sunday, Sage and Adam aren't interested in suppressing that kind of thing.
You should be paying more attention to the actual threat
All that said, you do have to be careful basing a class on the Witcher. File the serial numbers off, make sure that people can recognise it but you're not using any words, names, images, visual character designs, or anything else that is owned by Atari, and you should be okay. If you're planning on making any money off it, you may want to research your legal rights and obligations here. You can produce something based on someone else's work, but it has to be pretty substantially separated and only recognisable by people who really get the references.
A DW class PDF for sale on RPGNow based on the Witcher is highly, highly unlikely to draw Atari's attention, but you never know. They've sued people into bankruptcy for less. Your safest best is to not make it obvious that it is a Witcher, because if you use Atari's trademark to draw attention to your class, if you advertise "You can play the Witcher in Dungeon World!" as a feature of what you're selling, you've violated their trademark and you are—if they notice you—potentially in serious trouble. At best they send a Cease & Desist to RPGNow and they pull your PDF. Worse, RPGNow closes your account. Worst, Atari goes for you directly.
So don't get noticed, and don't do anything that will make anyone notice that your class is based on the Witcher until they've already bought it, and you should be in the clear.
Of course, if you're not selling it, then you can probably get away with saying you've made a Witcher class and here's the PDF, since there's a lot of precedent for ignoring fan-made stuff that's not for profit, but you still have to watch out, because you're on the wrong side of the law then, and you're only OK so long as the company a) doesn't notice you and b) turns a benevolently blind eye.
I think I see your confusion, and in retrospect, that is somewhat oddly worded given that Location and Dangers aren't really detailed elsewhere. I'm going to start from the root of this, so if one of these sections looks really basic and obvious try skipping to the next section.
Use a monster, danger, or location move
In Dungeon World, the GM is limited in much the same way the players are. When something gives the GM the opportunity to act (usually a player rolling badly) then the GM picks one of the moves from their list. The first of those (and probably the most commonly used) is "Use a monster, danger, or location move." This seems a little strange (we get a move that lets us get another move? What?) but it's exactly what it sounds like. When you use this move, you pick a monster, a danger, or a location, and use one of its moves instead of one of the other GM moves. The narration should be the monster, danger, or location doing something to present a problem.
You are totally allowed to narrate one of the other moves using a monster, danger or location! A monster could laugh and tell them the horde sacked the town last night revealing an unwelcome truth, a storm could get water into the supplies using up their resources, and a cave tunnel could collapse and separate them.
How would a location have moves?
When a location uses a move, it doesn't necessarily mean that the place has a genius loci or other animating mind. If you're making a location up, you can give it moves just like you'd give monsters moves. Except, there's a section in the book talking about how monsters have moves and how to figure out what moves a monster has, and nothing like that for locations. I never actually noticed that until I saw this question. Basically, think of things in a location that would go wrong for the players; in a bog someone might get stuck in the mud, on a mountain stones might crumble underfeet causing someone to fall, in a dungeon there might be traps that fire at the players. Those could all be moves. You can improvise them on the fly from the basic GM moves, but if you aren't a huge fan of improvising you can totally make some in advance. I've made an example below.
Volcanus Peak
(Magma)d8 damage. Tags:Magical, terrifying
Volcanus peak is a less-than-dormant volcano. It's a dangerous place even if there were no death cult intent on seeking the end of all things, but such is the life of an adventurer.
Instinct: To drive out interlopers
- Crumble beneath someone's feat
- Let loose flows of liquid flame
- Erupt in a terrifying display
That's not the best example of a move, but you can see how we're basically building a monster without hit points. (How do you kill a location? If you have an answer to that, by all means give it HP.) It does things the same way monsters do.
Alright, but what's a danger anyway?
You got it in one. When you make fronts, you also make dangers for the front. When you make dangers, you also make a couple of moves for those dangers. Check out the section on making fronts again- they give a whole bunch of example moves for different kinds of dangers (Planar forces, hordes, etc) and even a section on making custom moves if you need them.
TLDR
Basically, in Dungeon World some locations and all the specific dangers of a front have moves just like monsters. When the players give you the ability to make a move (probably by failing a roll, or by standing around looking at you) then you can make a general GM move, or a more specific move of a monster, location, or danger. Monster moves are detailed really well, danger moves are at least listed, and location moves don't seem to be mentioned again after that one line. If they are, I can't find it, and I'm kinda curious why I didn't get confused when I first read this too.
Best Answer
It isn't OK. I don't mean in the sense of “oh no everything is broken now!”, but in the sense of “yeah, Dungeon World is way less fun, and it will fail to be awesome in ways you wouldn't even know could have been awesome.” That sense of the game being ‘off’ is what Dungeon World feels like when its heart and guts are missing because some core rules aren't being followed.
Put another way, the game fails to live up to its promises when played that way. It can be played that way though — you just get a pale imitation of Dungeon World instead of the real deal.
How to fix it and make Dungeon World sessions gently unfold like a spring flower of awesome
This is something the players are doing, but the fix is super effective and entirely within the GM's control. It's easy to miss, but the rules say literally what the GM must say when the conversation takes this kind of turn, and the GM must say it in order for the GM to be actually following the GM's rules.
Here is it (p. 180):
It's that simple, but the GM has to be consistent for it to work.
Doing it in practice
Remember that Dungeon World's rules are all based on dictating legal conversation subjects. Weird, I know, but it works and it's important to remember so that the GM can see where and to what the rules apply during a session. This is one of those situation, because the players are attempting to skip steps. It would be like if the player of a 1st-level single-class fighter in D&D 3.5e says that they cast a fireball at the villain and start rolling 10d6. If that D&D 3.5e DM is all like “uh, okay, I guess they die” instead of “what, no, you're not a spellcaster” then D&D 3.5e isn't going to really work properly, right? Same thing here: when players just call out the moves and the GM acquiesces, Dungeon World doesn't work properly.
So in practice, when a player names a move and you say “OK, what does that look like?”, there are a few ways they might respond.
The best case is they pull up short mentally, think for a moment, and then start describing what their character is doing instead of what move the player hopes to trigger. This is awesome and lets the game roll on pretty quickly.
The middling case is where they're confused by the question and you have to elaborate. So you say something like “I mean just that: when you cast a spell, what does that look like for you, Cleric?” and “Well, you're hoping to spout lore, so what does consulting your accumulated knowledge about something look like for the Wizard?”
The key here is that you, the GM, are not allowed to contribute the results of what they do to the conversation yet. You keep asking question, talking about the fictional situation and what they are or would do, possibly resulting in events progressing in-game with no moves triggering yet, until a move (player or GM) is triggered and you have to follow its rules. If the player in question keeps trying to skip ahead and execute a move without triggering it, they're going to keep looking at you for the result and you're going to keep not conversing about the move, 'cause it hasn't triggered yet. Keep bringing it back to asking them to describe what they're doing, and watch for move triggers in the description.
Be prepared for the description to actually trigger a different move, because moves matching the fiction regardless of what move a player wanted to have happen is a powerful and core part of the feedback loop the whole game is designed to create.
The worst situation is where they argue, especially if they pull out a half-baked understanding of the game and claim that Dungeon World lets them do anything and the GM has to go along with it.
You're still the GM, and the world is basically in stasis until the GM converses about it changing, so misguided, arguing players can't really force the game forward without your cooperation. That means: Don't Panic. It's fine, the game is fine, and you just need to chat reasonably with them and rest confident that the game reality will wait patiently for you. There's no rush, so you've got time to correct the misunderstanding without actually getting into a heated argument.
In practice argument is actually unlikely, but it's good to have some ideas for keeping the game from being sucked into the Argument Black Hole. So there are a few ways to handle an argumentative player.
But all this is a long-winded way of saying that the GM's job when someone just declares a move's name without actually describing their character doing anything is to somehow get someone — anyone — at the table to tell you what they're actually doing in fictional terms. If it's the player who just named the move, that's great, but there's no initiative and you can flip over to someone else who'll help you keep the conversation about the fictional events moving instead of stalled.
Get people saying what they do in game, and only call moves that match after they match the triggers, and Dungeon World will deliver the subtle awesomenesses it promises.