Don’t hold back!
It’s a lot more fun to win despite adversity than through the sheer lack of it. That’s a very important lesson learned from playing with Eric and Bastien.
I see this question a lot of times on the net: “How much can I throw at my players in Dungeon World?” Short answer: All you’ve got!
Dungeon World really empowers the players, sometimes to a scale where the GM can feel a bit powerless. If they continue to roll well, they can more or less do whatever they want, as long as it does not contradict the fiction.
As the principles goes, we are supposed to be a fan of the characters. We are doing that by letting them be awesome. They are awesome when they win despite the odds, not because of them. Also, we should think dangerous but we shouldn’t limit it to thought.
Keeping a hand over the character only blocks their limelight. Think about your own campaigns; which moments were the most exiting? The ones where the players leveled armies without breaking a sweat? I’m guessing the answer to this question is a big resounding “no”.
In my last game I actually had a player roll Last Breath. Just telling a player to roll that made me shiver. I really wanted to make the roll meaningful, and if he had rolled a miss instead of a hit, I’d probably had let him be the one that toppled the tower over, sacrificing himself to save his reality. That would have been an epic ending, but I’m still glad both characters survived.
Without danger, adventure is meaningless. Don’t hold back, and give them all you’ve got! If you know that the characters will survive, then they are not really in danger.