Galeros said:
I hate to hijack the thread, but could you give some examples?
I see some other posters have already started work and offered some good stuff. Here are a few more examples:
1. The idea that people's relationship to pagan gods would be something like people's relationship to God in monotheistic faiths.
2. The idea that feudal states will have mechanisms that can enact or enforce laws the way we do in modern bureaucratic states.
3. The idea that gods cover a portfolio (e.g. rain) in a neat and rational way.
4. Free, unconditional purchase of services, land or goods in exchange for simple cash.
Anyway, this is just off the top of my head. There are plenty more.
EDIT: I'll disagree with DannyAlcatraz on one thing: racism. The concept of racism did not make a whole lot of sense to medievals. Despite the fact that they spent much of their time, from the 7th century onwards fighting Arabs and Africans, the idea of race did not emerge until the 16th century. "Race" as an idea, is a fundamentally modern notion.
People were hated because their religion was different, their fatherland was different, their culture was different, etc. But the idea of hating people simply because of how they looked is pretty new. You'll note, for instance, that in Parzifal, one of the original Holy Grail stories, the hero cannot achieve the grail until his black half-brother comes to his aid. One of the favourite saints of the Middle Ages was Saint Christopher, who was believed to have been a dog-headed man.