This is a follow-up to my "Wisdom from Campbell" post. It occurs to me that, while in practice the use of the priestcraft-and-enthusiasm locus is very vague, and more social in application than anything, both of the words seem to have something to do with way in which revelation is mediated. In 'priestcraft' revelation is entirely mediated by a magisterial hierarchy; in 'enthusiasm' revelation is entirely mediated by personal inspiration, conscience, or inner light. This is likely the origin of the Scylla-and-Charybdis notion; the establishment churches of Britain would have seen themselves - in different ways, obviously - as avoiding either extreme. The social aspect would have come from the actual groups that would receive these labels, and thus would tend to indicate those advocating, from a religious perspective, either hierarchialism (if that's the right word for it)or libertariarianism, in contradiction to the establishment compromises. Thus Quakers, of all people, could be seen as dangerous to society (which some people certainly did consider them to be): their religious views, springing from their notion of 'inner light', would have been seen as a religious excuse for anarchy.
Such is my hypothesis, anyway. The facts I build it on are 1) the way philosophical thinkers in the period actually use the topos, and 2) the sort of conclusions they draw from it. But I'm sure there's some intrepid historian out there who, from the more purely historical side, has touched on this distinction in some way that would allow for a more precise filling out (or correction) of this suggestion. If anyone has come across anything in this regard that might be of some use, let me know.