Wouldn't change the games much? I'm having a hard time wracking my brain for a game I've played that wouldn't be changed much if it were set in the dark ages. Pretty much every game that is played here is about 2 parts social/investigation, 1 part combat. Some are majority combat, some have no fighting at all (but are still dangerous, of course). A lot of games take place in cities or small towns where any change in culture or technology would make those games impossible to run. This isn't a bad thing, of course, since I think games should follow naturally from their setting and characters; it just means that new games would have to be made.
It would also change the abilities available as well. Goodbye firearms, hello archery! Engineering and science would be massively altered as well. Each setting would probably warrant new major abilities on the character sheet, or at least massive changes to the current ones. I can imagine medicine being a lot worse without magical aid in the dark ages. Much worse indeed.
