Mouse Guard didn't bring me out of the imagined scene to be a dramaturge the way that Shock or PtA does, but it did bring me out to deal with mechanics and rules more often than I'd like. (Many 4E folks seem to really like Mouse Guard, but perhaps for reasons of realism the BW game-family wants you to spend a lot of time looking up which things FoRK with which other things etc., while one of the things I like best about 4E is that you never have to look up rules, although you may spend a lot of time scrutinizing your character sheet).
This seemed to happen most often when we were at a zoomed-out scale (how can we get this grain wagon back on the road?) rather than in combat, where my action was usually my own & didn't require reinterpreting based on other players' rolls and mechanics that got called in. That supports two things you were saying - that the less concrete and momentary the action, the fuzzier its resolution, and also that we're super used to the backsies of "I hit the goblin"/"No, actually you miss" so it goes unnoticed.
The other thing Mouse Guard and Shock both had was that as a player my motivations weren't always aligned with my PCs, which I dislike (because I'm an immersionist?). In Mouse Guard it's at least clear that I'd work against my PC now in order to get a benefit during the GM's turn, which is bass-ackward to me but at least makes a kind of gamist sense. In Shock I felt like I was simply making decisions (like "do I interfere with another player's conflict") for aesthetics alone, which is not compelling to me.
Maybe my feeling is that I liked the initial setting-making more than actual play because I feel like the existing tools I have in a normal conversation to hash out ideas and aesthetics are enough, whereas freeform storytelling with Javi has convinced me that it needs extra tools in the form of dice to handle "I shot you"/"no, you didn't"?
