How do I explain to players why there are no situational modifiers in Dungeon World? Being used to other RPGs, they would expect modifiers for performing actions in a more challenging circumstance.
The Apocalypse World engine basically isn't concerned with difficulty on a short term basis. Generally, the math slightly favors player characters at all levels - a 7 gets you a decent result, and 7 is the natural result you're looking at as the average of 2D6 if you have no negative modifiers.
So where does the difficulty come in?
In the shorter term, the difficulty comes in how the GM makes Soft & Hard Moves and decides to frame the players' Moves. If you want to make anything harder, it comes down to giving the players harder choices or stuff along the lines of a step or two that might require Defy Danger. ("His cover is too good. You want a shot? You're gonna have to flank him. But that means leaving your own cover to get to it...")
In the longer term, the players will face the most threats of play - dice roll after dice roll of danger. Eventually, somewhere, you fail. And then things hurt. Not just hitpoints, but gear, or social rolls, or getting split up... and those things create new problems. It won't be a single problem, it'll be a failure in a situation that creates a cascading problem.
See, when problems KEEP coming, even having a 70-80% success rate isn't "too easy", it's just a matter of time when those problems finally kick in on you. And hurt. Eventually, difficulty catches up to you.
Well, if you really want, and it fits the story, you can apply some modifiers. That being +/- Forward, or +/- Ongoing.
That being said, Dungeon World is more general than specific. You can use the conditions to explain a "Failure" or "Partial Success."
For example, Roy the Awesome Ranger, decides he's going to fire an arrow, through a dark tunnel, at the approching light. He:
Rolls an 11. "He's on fire... or at least his arrow is, as it passes through the goblins torch and into the body of the beast."
Rolls an 8. Chooses to lose some ammo. "It was dark down that tunnel, and you couldn't see some of the outcroppings. A few arrows struck some rocks, but as the light approached, and you began to see the gleam of feral teeth shining in torchlight, your arrow found it's mark."
Rolls a 3. "Whelp... that did it... Apparently the light has decided to go away from you now... because that arrow just embedded in the scales of a nasty looking Rock Serpent that was hidden in the dark... He's uncoiling quickly, and he's coming right at you.
Then, all the little things become story more than mechanics.
In D&D (and most other games), the dice are rolled to determine whether the character succeeds or fails. In Dungeon World, the dice are primarily rolled to determine which player (including the GM) gets to narrate the result, and what kind of things they can say. This is a subtle distinction, but understanding it helped me, at least, get a much better handle on what the DW rules are for.
Dungeon World isn't D&D (or any other RPG)
It's just supposed to have a classic fantasy "feel" to it. I think everyone else covered things pretty well, but I think the critical thing you're missing in thinking about this is that Dungeon World rather explicitly wants failure to be interesting. Not only is that when GM moves happen, but it's also when you gain XP. Everything is tied into this idea of story, then roll, then story; which means that by adding circumstance modifiers you're actually messing with the whole system.
It's also worth noting that if the narrative suggests something should be impossible (Volleying a target through a solid wall, Hack and Slashing toe-to-toe with a Tarrasque, or Defying Danger through an army focused entirely on you to reach the general) you just can't do it. This shouldn't come up often, but when it does it's critical to remember this rule. Likewise if something is unlikely to fail - especially in an interesting manner - (throwing a torch into the next room, a coup de grace against a hostage, climbing a young and sturdy tree) it probably doesn't trigger a move in the first place; no roll required.
A starting point for explaining this could be that you only roll in challenging situations. A move is triggered when the players make a decision to act in a way that will have interesting consequences whether they succeed or fail. And no matter how calm things seem right now, an outright failure is going to result in the GM making a hard move, which can quickly begin a slide into mayhem.
So rather than having a "can I open the door" dice roll, we assume that if there is no particular danger there then we can just walk up and open the door. There might be a trap there, of course, in which case your thief could decide to search for one, triggering the Tricks Of The Trade ( risking failure ) or the person opening the door might need to react when the trap is triggered and Defy Danger. Perhaps the door is locked and the Fighter might choose to bust through it, triggering Bend Bars Lift Gates. All of these are moves that have definite stakes and consequences to failure, so you roll them straight.
If things get difficult then what changes is not that individual rolls necessarily become more challenging ( although that is at your discretion as a GM ) but instead that more moves are triggered. So if you want to run across an empty room, no moves are going to be triggered, if you want to run across a room full of angry armed goblins then you're going to have to make some fancy moves to arrive at the other side safely - you're sure to need to Defy Danger a time or two, probably in different ways, maybe you'll have to Hack And Slash your way past them. Every time you do, your chances of hitting a failure increase and when you fail ( or even hit an intermediate success ) then things start to happen that impact your plan. Maybe your sword has broken, maybe one of the Goblins runs off with the treasure you came here for in the first place, perhaps they have a cave troll!
These failures and the interplay between failure and success are what drives the story ( and consequently the action ) in Dungeon World. A failed roll isn't "you can't do it," it is "now something bad is going to happen and you are going to have to decide what to do about it."
A closing point to explaining this might be that it's not really a question of how difficult something is, it is a question of how many ways it goes wrong. The rules aren't telling your GM to think awkward, it is their job to think dangerous.