I've been integrating elements of doubt into the 10th Age since I began designing it a few years ago. For doubt and uncertainty form the core of our experience as human beings and I would feel remiss if I didn't include that uncertainty in a setting meant to emulate life (albeit a very fantastic life). At the core of our experience there is an eternal questioning, and enfolding that questioning into a game is an effort to recognize that and to reward it.
While types of existential doubt are manifold, the type of doubt I'm talking about here is something more basic. We generally assume that we can obtain information in roleplaying games, that the information will be correct, and that if it is not it is because someone is deliberately misleading us. This is a straightforward approach that is reminiscent of the modernist way of thinking. There is a certain truth, and the road to uncovering it is one that is generally linear; any potholes that we hit on the way may delay us, but they in no way invalidate the central the fact that our journey has a destination.
Maybe it was art school. Maybe it was reading Allen Varney's Paranoia. It was probably some of both. The notion of an unstable truth has interested me for nearly a decade, and I've worked these unstable and uncertain histories into the 10th Age. There are great mysteries that can never be answered at work at the heart of the setting, and that lends a certain mystique, a reality, to the world. And mysterious questions aren't the only manifestation of this tendency: there is conflicting information scattered throughout the setting. This is information which defies simple synthesis. Not all evidence will support all claims, forcing characters (and players) to pick and choose what evidence is relevant and what must be discarded or denounced.
Of course, not everyone needs to play on this level. The setting works just fine as a place where people pick up swords and kill one another. But given the option, I would like to think that these irreconcilable oddities make it a deeper and more realistic place.
There is another way in which these doubts function, also, and that is to help recreate a society prior to the homogenization and standardization of mass communication and publishing. Spelling inconsistencies are rampant because there is no single corpus of recognized spelling and grammar (outside of the elves, who have a homogenizing cultural tradition in the form of their paidea, the training regimen of classics that all elvish children learn). Not everyone believes the same things and not everyone even believes compatible things. There is as strong sense of localism and regionalism, because the ties that bind us in large geographical units in the modern world are weak or altogether missing in the 10th Age.
I think that doubt can be an important element of any campaign setting or game system, AD&D or otherwise. It is comforting to remove it, to give concrete answers, but it may be more rewarding to include it.