Its interesting. The tradition of the farmboy is fairly well established, but what is more interesting than that is the interpretations that have been put on it.
In Arthurian Romance, anyone of common descent that achieves somthing great is not really of common descent. it always turns out that they are a lost line of nobility that has be rediscovered and is no longer on hard times.
Also, fascists have used the farmboy mythology to their advantage. In fact they have traditionally borrowed from the Arthuruian Romances and pastoral mythology. As most fascist movements come from rural agrarian areas (esp Germany and Italy) the farmboy with destiny is the poster child.
It comes from the idea that the farmboy is not corrupted by civilisation, and that he has some legitimacy in saying how things are done because of that lack of corruption. He shows up and saves everyone and puts the city in order. The low extreme of this is Conan. The high lofty extreme of this is superman. Conan is not civilised and so is able to govern aquilonia well because he does not have the corruption of civilisation in his blood. Superman is simply not human. So he does not have human corruption getting in the way of doing the right thing.
The farmboy indicates purity and the saving the world/being the king indicates higher capacity than the common man. But his purity allows him to relate to the common man. Basicly the structure of savior requires that the person be uncorrupted and extraordinary. Aragorn fits this mythology also. He is of a higher race than normal men, but he has been born into common trappings.
I would agree that there are not nearly enough stories of saviors that do not follow this pattern. Elric is definately one of them. The lankhmar stories are posibly another. I think we need more common people saving the world but still being common.
It has been said that D&D is its own genre. Perhaps this is an aspect of it. That in most campaigns and novels, the PCs while they have powers (that more often then not are not extraordinary), are not pure or of a better line than anyone else.
Aaron.