Great discussion lads :)
My input after all has been said is a commentary on GM 'style' or behavioural response through moves. Defy Danger is the 'goto' move by the players for sure, but only after elucidated through the fiction. So the choice of soft or hard move by the GM in determining the 'danger' is all part of what Chris refers to as fiat - but its a choice by the GM based on the fiction - its the framing of the danger by the GM that matters too. Even if the GM makes a hard move, with the consequences metered without player adjudication, it is never in a 'vacuum'. Everyone is excitedly narrating at this point, moves are being made, dice are being rolled.
If the danger is threatening the players but hasn't effected them yet (the GM is setting themselves up for the hard move to follow) then the GM move is presumably soft. If however, the danger has already effected them or is ongoing, then the players are probably responding to a HARD move, which is most suitable on a failed roll or total ignorance of a sign of doom.
The consequences of the danger seem to be problematic; especially in combat/damage scenes where the age old (war)gamer habit of dissecting the fiction into 'turns' seems to muddy the issue. I think everyone's primary advice here is to follow on from the fiction. A 'hard' move doesn't mean random, irrevocable badness happens. It means totally focused, re-incorporated, established and omnipresent badness happens and effects you, so now what are you going to do about it?
So in our play, we've found defy danger can be both; either reacting to the threat of something happening (softer GM move). Or responding to an established danger after it has already effected the fiction (harder GM move). How and why the danger has been introduced is always deeply embedded in what has already been contributed to the story.