I have been thinking more about different kinds of adventures. I realized that I have played quite a few games, for example Beyond the Mountains of Madness, where there's obviously a plot. Those kinds of game can be horrible, in a bad way, if all you get to do is to look at the NPCs doing stuff.
But, thinking about what makes Dogs in the Vineyard work, I realize it is not only the fact that the PCs are clearly in charge, and have something to do (even though it helps) but one thing that makes a difference is that the NPCs all want something. Maybe even a monster in a dungeon wants something, even if it's just a slice of that pie? BtMoM was about wants.
Not exactly revelations of deep design insights, I know. But, I somehow feel maybe it should be more up front. The first thing you get to know when you see them is what they want, not that they guard that door or treasure over yonder.
Maybe it's just be that suck at NPCs that have finally caught up with the rest of you...