NOTE: Way back in early June, before I saw the post on Grognardia, I had worked this up. Unfortunately it remained unfinished, my last crack at it being the 12th of June. It remains a source of embarrassment that I was too busy at the time to finish it. New job and all that brings.
The Cleric is a misunderstood term used in D&D and quite a few other games as well. About the only thing 2nd Edition did right was to come up with a better name than "Cleric." And then 3rd and 4th editions came along and brought that retarded term right back to life. "Druid" and "Bard" are also terms I have issues with, but Cleric is chief among them.
Let me elucidate:
The term "Cleric" is not a generic religious term. On the contrary, it is a highly restrictive term. A Cleric is a Roman Catholic term, from the Greek klerikos (an acquisition). Being a Cleric meant you had to have your head shaved, leaving ring of hair around a bald scalp. If you didn't do this, you would not be considered a member of the clergy. So, if you don't consider your character to be a literate dude with a robe and a funny haircut, you're not really playing a "Cleric."
"Cleric" eventually became "Clerk," which is why bookkeepers and such are called that today.
"Priest" would be a better term, as that is a generic term that can be applied to any religious leader of ceremonies. Though it, too, has its issues, it tends to have broader applications.
"Templar" and "Hospitaller" might even be accurate, considering that they were martial orders. True, I know those are both specific Christian Orders, but bear with me. Templar referred to the Temple of Solomon, but in the game it doesn't have to, it could be a temple of any religion you like. The Knights Hospitaller were also referred to as the Sovereign Military Hospitaller Order and as the Knights of Malta. Either of those, especially shed of Saints and the Hospitaller reference could be used. Instead of Knights of Malta you could be known as the "Holy Knights of Dawnmoor" or something more fantasy-related.
Druids are next on my list. There's nothing wrong with the term, per se, but you'd want a bit of historical perspective, change the class description to something more in line with the established historical records, such as accounts by Strabo and Diodorus Siculus. According to Julius Caesar, they were involved in "divine worship, the due performance of sacrifices, private or public, and the interpretation of ritual questions." Fairly well like any other priest. However, druids were much more than that. They kept the peace between nations, and served as judges, counselors and advisors to kings. Their truck with deities seems to be different from what we would consider normal today. It is almost as if they consulted the gods and considered them wiser peers, than the absolute division between god and man that exists in more traditional Western religions. In Greek and Roman writings there is evidence for druid sanctuaries, mistletoe and plant rituals, sacrifices and reverence of the four elements. The main difference between Druid and Priest (Cleric) is that the Druid was intertwined in many different aspects of Celtic life, whereas the Priest was set apart from (and indeed above) the citizenry.
The Bard was, according to Caesar, related to the Druids, merely not mentioned by name. In one text published in 1918 (which I cannot put my hands on and therefore have forgotten the title), they were detailed as being educated along with the ovates and druids all together, and that Druid was a career path as well as Bard. Authors such as Festus, Lucan, and Ammianus Marcellinus indicate that "Bard" was a respected title, and they were the official poets among the Gaulish and Briton Celts. Summoned by royal commission, it was their job to celebrate the victories of their people and to sing songs of praise to their gods. They were important to the people, and the sense of national pride together with religious belief gave Bards a powerful influence over the hearts and minds of those they sang to. They were also steeped in the traditions of clan and country, the historians of their people, since no written records existed, only oral tradition. They were also skilled satirists, some of their satires having the same effect as a curse.
Hopefully this information can be used by a GM or player to really lend some background to these somewhat misused terms. I'd love to see a game come out that does justice to these classes and places them in their proper roles.