High fantasy RPGs tend to be set in an era similar to the late Middle Ages. This seems to be the norm for D&D, and it seems to be what players of such games expect.
Let's say you have a group of players used to this type of setting. Your new game is set in the Iron Age among the Celts and Germanic tribes, somewhere between 800 and 200 BC.
What's going to be the most jarring aspect of the setting for these players?
Best Answer
Things I would emphasize in an Iron Age setting:
Lack of information. In medieval settings, while peasants might know rather little of anything beyond the next town over, scholars at least have a pretty good idea of "the big picture". Just one example: Maps of the continent you're on exist, and while usually being pretty bad as far as scale is concerned, are more or less correct on the existence / non-existence of things. For an Iron Age setting, assume that any map - if such a thing even exists - has significant areas of white, and would have a very high chance of being simply incorrect. This need not be the "sailing off the edge of the world" thing, but more the general uncertainty of things. What's beyond those mountains? Dunno, no-one ever bothered to find out. There be dragons, me granny said.
Unrefinedness. No Sword Dancers, Duelists, Samurai, Warrior Monks or some such, but a more basic selection of Fighters, Rogues, Thieves. The same for magic user classes: Nothing refined and elaborate, but magic on a more fundamental level. Little use of quick spells or power words, more use of elaborate hour-long rituals and meditative trances. (See next point, too.)
Superstition. While most medieval-setting RPGs handle magic quite casually, in an Iron Age setting I would picture magic as thing of the gods / demons / the devil. Reverred shamans or healers, dreaded conjurers of demons, and very little in-between. If you allow magic-user PCs at all, any overt display of magic would be a "big thing", and could get them in trouble any which way: You might end up with a mob on your heels. Whoever is in power will want you on his side, or dead. Any way, you will be perceived as otherworldly, shunned, avoided, whatever. This might be taken to extremes, with magic being from the gods or demons, and wielding it being possible only to those being in servitude of either power (with all consequences). What will not happen is that you're accepted as "a guy with a different set of skills", or casually visiting the local magic library. If you have magic, you either set yourself up as the local shaman or priest, seize power and become the bane of the region, or you take pains to keep your magic subtle. One open incantation, a single fireball in front of witnesses, and you're blown, my friend.
Item availability, prizes and monetary system. Some things are simply unavailable in certain areas (perhaps even utterly unknown), others hideously expensive. (Cloths, jewelry, certain metals / alloys, certain tools etc.) Money exists, but barter is still the rule, especially in rural areas. (Money isn't the ubiquitous commodity, but still actually seen as, literally, "a chit from the king".) Few or no settled merchants, trade mostly being done by peddlers. (Personally I recommend "...and a 10-Foot Pole" from I.C.E., which has price / availability lists plus economic background information on everything from stone age to post-modern.)
Edit: And one thing I completely forgot, and rediscovered only when perusing the Rolemaster material on the subject...
Slavery.
Transscribing from RM material:
In a primitive (stone age) society, people live on a subsistence level, as hunters / gatherers, slash & burn farmers, or fishers. There is both little need and little space for "having slaves".
In a feudal (medieval) society, slavery might still exist (and did, in our world), and many live in slave-like serfdom, but most people are "free".
In an antique (iron age) society, however, slaves can make up a substantial percentage of the populace, to the point of actually being the majority. This will certainly color the environment, and makes "runaway slave" a background option worth considering.