It's easy to forget that people aren't the same all over. After all, we're all people, aren't we? This is a common error made by historians when approaching the past; we tend to take for granted little social cues and learned responses and assume that everyone has always had them, that everyone has always behaved more or less like us. That couldn't be further from the truth! In history, we must attempt to reconstruct lost social identities in what is called histoire des mentalités, the study of cultural mindsets. We must constantly remind ourselves that we take our entire worldview for granted. Without proof that people in the past thought just so about a certain subject, we cannot assume anything about it. We need to reconstruct, carefully, the elements of the past that make up cultural norms. When we find someone in some piece of history that thinks like us, we can crow and dance and be happy that everything perhaps isn't quite so alien—for example, the riddles of the Exeter Book, which are mostly veiled penis jokes.
We cannot simply ignore this fact when we're making settings for roleplaying games that are not based on our own time. Well, we can, but we do it at our peril.
Cultures that are not based on the same norms are foreign to us and we cannot afford to forget it. This is one of the reasons that I write so much background material for my settings. I know it's a terrible burden to read all that backlog, which is why I am also working on a digest of it in the form of a player's guide. Accessories of this nature can help us by explaining those strange little quirks of culture and mentalité that we just don't understand.
It is deceptively easy to fall into a pattern of modern thinking when dealing with premodern societies. We have a lot of things that we take for granted, and backport when thinking about the past. It would surprise you to know, possibly, that in the classical and medieval period all glass that was used in Europe was actually made in large glassworks in Egypt. This meant that, after the collapse of the Roman tax spines, there was very little glass in the European countries until after the Crusades and a re-establishment of glass-making technology in the mainland.
This means that throughout a good portion of the middle ages (certainly until the late thirteenth century) there were no glass window panes. Yet, trade still existed with India which we know for certain because garnets, which are only found on the Indian subcontinent, continue to appear in Anglo-Saxon jewelery.
What's the point of this? Well, it's that we need to be very specific when talking about times and places that are inherently alien to us. There is so much to know and so much that we generally don't care to find out when playing a roleplaying game. Sure, roleplaying games are supposed to be games and thus are fun, not work. But I remember when roleplaying was both fun and a hobby. Hobbies generally have an attendant amount of effort that you put into them; whether it's building a model airplane or rebuilding classic cars, you learn things and develop knowledge by practicing them.
Fantasy has always had the accusation leveled against it that it tends towards the generic. "This setting is just like the last setting," syndrome has plagued it for decades, and in the lowest bracket of fantasy writing this is true. I would argue, however, that fantasy literature is no different than any other branch of literature. Sure, things of a similar nature have been discussed before, but the devil is in the details, as it were. You don't necessarily need to have an entire crop of new never-before-seen fantasy races in order to have inventive fantasy. That's one of the policies I've hewed to very closely with the 10th Age, in fact. It is prototypical to the extreme, the ur-fantasy if you will, but it has a very detailed and precise set of cultures and history.
I think one of the best examples of this in the past few years it the Prince of Nothing series, which very clearly resembles the late classical world. Yet Bakker goes through great lengths to make sure that things are not too familiar. He alienates them, others them. George R. R. Martin did the same thing in his Song of Ice and Fire when he decided not to use the English "sir" as a title for knighthood, but rather the Italian "ser." What effect could this possibly have, you may wonder. In English, the use of "sir" has become commonplace and can be used in reference to just about anyone. In ASIAF, its use clearly denotes a title that was earned and thus, by othering it, Martin causes us to notice its cultural value. (I'm not ashamed to say that the 10th Age makes use of the French "sieur" based on the fact that the Mileans are somewhat Frankish in style).
In short, understanding and inhabiting the culture of your setting will not only give it more depth, but it will allow you to become involved in your game on a level that is more interesting than the racial powers you have been granted by the system. Steve Winter brought this up in an essay he wrote for Kobold recently. Take the time to step outside yourself and understand the culture you are playing in, and you will be rewarded for it.