This seems to have become both more and less clear as time and fluff has gone by, simply;
Do Priests of the Old World Pantheon have access to the magical powers of their gods? Are they effectively wizards?
Fluff-wise many priests are described as magic users (Bagrian - High Priest of Taal, Morgianna le Fey - Enchantress of the Lady) and the powers of the gods flow from the Winds of Magic, exactly the same as spells from a sorceror or shaman do so there is no difference in effect.
Is there a difference between prayers and spells?
Equally some priests (particularly Sigmarites) have no magical talent at all, faith is seen as seperate from magic and divine intervention is open to anyone with obsessive belief!
All priests can supposedly call on their god, the god in question does not have to listen. Are magic using priests the exception or does their seniority grant them the absolute attention of thier deity? AND access to the power that comes with it?
If actual magic users are the exception in the clergy, how welcome are they? Do they fulfill other roles? Could a High Priest also be a member of the colleges of magic?