I think I finally figured out what makes Jon Huntsman the adult in the room. If you look closely at the image below, when the candidates were asked whether they'd reject a debt deal containing $10 in spending cuts for every $1 in tax increases, he raised his hand less enthusiastically than any of the other candidates with the potential exception of Ron Paul.
Notice how every other candidate has hiked their hand higher in the air? (It might be a tie with Ron Paul, but if you look at the elbow, Huntsman's elbow clearly is a tad lower, so I give him the edge.)
Of course, it's true that Huntsman actually was embracing the exact same position as every other candidate on the stage: He'd reject a deal with a 10-to-1 ratio of spending cuts to tax hikes. And it's true that on just about every policy issue other than civil unions, Jon Huntsman shares the exact same policy view as the other candidates on the stage. But he's not as much of a dick about it. And that makes him the adult in the room, even though he's doing the exact same things as the kids.