The medieval mindset is very different from the modern mindset in some key ways. One of the most important ways is that it was far more uniform. Diversity of opinion itself is a relatively modern concept, and was much more circumscribed in the Middle Ages. Here are some key social tenets that were nearly universally believed in the Middle Ages that modern people often have trouble with.
- Nobles are different from commoners. They are genuinely better than commoners. They may have flaws, but even so, the flaws are exceptions.
- Marriage is the core unit of society. The purpose of marriage is to create a family -- that means children. Those who reject marriage put themselves in the same fringe of society as occupied by harlots and thieves.
- Worker's rights? What are those? The vast majority of people barely subsist and consider themselves lucky for it.
- There's no such thing as free trade, and the idea of laissez-faire economics is absurd. Monopolies are not just part of trade, they are almost all of it.
- Social mobility is a fairy tale. (Literally. Think back to the fairy tales you learned as a child. How many of them, especially the oldest, talk about the incredibly unlikely or even magical means by which a peasant leaves the peasant class?)
- You can't hope to "deserve" fairness and justice. When you get them, consider yourself lucky. That's the exception; most of the time, might (and wealth) makes right.
- Almost everyone is, by modern standards, ugly. Between poor hygiene, rampant disease, vastly higher costs for clothing (and even more so for colorful or luxurious clothing), and long work under the sun, people are drab, wrinkled, pockmarked, and plain.
- Beer and ale are mainstream things everyone drinks. Water is often unsafe, but beer is more likely to be safer. And beer in those days is far weaker even than American beer today; distilled spirits strong enough to get drunk off are too expensive for you. Children drink beer and are none the worse for it.
- Everyone professes a religion. Someone who does not is not to be trusted, and perhaps seen as somewhat insane.
When everyone in a game deviates from its setting's social norms, those norms start to get ephemeral and feel unimportant. It's important that a lot of players make a point of embracing most of these norms. It's fine to deviate from some of them to make your character more interesting, but you should also "do your part" by embracing others to help establish the norms. After all, if no one established the norm you're rebelling against, it won't be very interesting for you to rebel against it. So help other people get that same frisson by giving them bits of the social norm to contrast with when possible.