I'm running a campaign (D&D 3.5, moderately houseruled) set in a world in a classical era. You might compare it to the Han dynasty, or to the Roman Republic–Roman Principate (Sulla to Trajan), in terms of technology. Glassblowing and horseshoes are recent inventions; plate armor, stirrups, spurs, mills, paper, eyeglasses, and compasses haven't been invented. (Well, actually dwarves have developed techniques for forging plate armor, but their methods are secret and the armor is not widely available.)
More important is the general lack of communication: while perhaps the elite have access to magic which allows them to contact faraway lands instantly, the vast majority get their information slowly and never travel far — travel is expensive, slow, and dangerous. This also means that the players' knowledge of geography is greater than the vast majority of the world's inhabitants: probably you'd know a bit about a neighboring kingdom, maybe you've even heard of the one beyond that, but you wouldn't even know of a country on the other side of the continent.
But it's a common assumption in fantasy roleplaying games (especially D&D) that the world is set in a late medieval period, if not Renaissance. And frankly, I haven't done a great job keeping up the feel. What can I do to keep the feel of my world distinct?
Best Answer
Name things in accordance to your setting. That is names of PC's. A Germanic name as 'Feldgar' sounds medieval, while a name as 'Claudius' sounds Roman. But also names of objects, e.g. use 'gladius' instead of (short) sword.
Use imagery. For example, use a printout of the floor plan of an actual Roman villa, of Bathhouse, when describing the entrance into a city use a printout of Roman city gate, or when talking to a priestess put a picture of a Roman priestess on the table.
Use situations, persons and sites specific to your setting. For example, Roman and Chinese cultures were quite bureaucratic unlike medieval Europe. Use that in one of your adventures. Have them deal with red tape. Have them bribe officials.
You probably did already, but remove as much as possible things that do not fit in your setting. No bastard swords, longbows or hand crossbows (they might have existed, but they do have a medieval flavour).
The main way of making players aware that some things are rare is by mentioning that explicitly when one of those rare things to turn up. The first thing to describe when they see a dwarf covered in plate armour, is the plate armour, how special that is, and how impossible it would seem to them that he can move in that.