This is a small idea I’ve been batting around. Let’s see if it makes sense.
A game is, among other things, a system for manipulating the attention of the participants. Games I like, I’ve noticed, tend to move your attention back and forth through the fiction and the mechanics, sometimes by convincing you that they’re the same thing (such as with Apocalypse World‘s “to do it, do it”).
But this doesn’t say anything about where the locus of interaction is. The thing you have to interact with most becomes most salient, draws your attention more strongly. I think I first noticed this with the draft of Et in Arcadia Ego that I brought to Metatopia. While failure and success occurred with roughly equal probability, the failure engaged much more of the mechanics, making it more salient, and making the game feel punishing.
Broadly speaking, a game can put more of the interaction in the mechanics or in the social arrangements. RPGs, generally, put a lot of the in-play interaction with the game in the social arrangements, compared to, say, board games or card games. Some RPGs move more towards the board game end of the spectrum, some board games (such as, say, Apples to Apples or Monopoly) have more of their interactivity in the social arrangements.
So, what, then, do these different parts of a game do for you?
I think that the social arrangements are much more conducive to technique-oriented play, and story, and, frankly, predictable outcomes. The mechanics—by which I really mean crunch, the things with dice and tables and numbers—are more conducive to constraint-oriented play, and surprising-within-constraints outcomes.
Somewhere in here, the issues of judgment calls and interpretation fit. I think that judgment calls are, fundamentally, a mechanic in the social side of this space. They’re bound by the group’s ideas about what is right. But interpretation, oh, it is a wonderful thing, because it exists at the border between crunch and social arrangements, and as such, it can be a great locus for context-appropriate outcomes, which are what stories need, and surprising outcomes, which is one important thing that games need.