One of my players recently expressed an interest in joining a mystery cult. Anyways I was wondering what difference, if any, there is between a mystery virtue and a hermetic mystery virtue (I can't find definitions for either in the book).
Technically, a mystery virtue is a virtue gained by a personal sacrifice, a story and an initiation. A Hermetic mystery virtue is a subset of the above, suited solely to Hermetic magi. In the RAW, I can't think of any non-Hermetic mysteries (actually I can talk about Ex Misc now, so there you are - a fair few of those can be learned by outsiders). So, practically the two have, in this edition, meant much the same thing, although now we are beginning to see the ermergence of non-Hermetic mysteries.
In the previous edition, for example, we had the Swan Knights, which were a mystery cult that had nothing to do with the order or even magi, but who gained their virtues through the mystery mechanic, which is that you earn a new virtue with a sacrifice, a story and an intiation.
Actually even in this edition the concept of mystery cults outside of the hermetic system is mentioned; there is a hint in RoP:TD that some holy traditions might be able to initiate divine methods and powers. It seems that the rules for magical and divine powers are similar in other respects, so no SG who has The Mysteries and RoP:TD should have any problems in working this out.
Also some faerie/magic gods (of whom I hope to hear in the missing ROP books) probably have cults centered around them which can initiate appropriate virtues. IMHO the stressing on hermetic mysteries in the existing books is just the fact that hermetic magi are in the centre of the game, especially in the books published by now.