I just aborted an attempt to read a novel (which I shall not name) based in "Medieval Germany." No century was specified. Seriously? By the most conservative reckoning, the Middle Ages lasted 600 years. Shouldn't a historical fiction novelist narrow it down a bit?
And then there was the typical problem of language. The novelist had the main character speaking (and thinking, since it was first-person POV) in a kind of Shakespearean knock-off. Remind me, what part of the Middle Ages did Shakespeare live in?
One of the issues my, er, purported publisher has with my medieval mystery novel is that its language is too modern. I have tried to impress upon him that, in early 13th-century England, people weren't speaking modern English at all, but Middle English, the language of Chaucer. And it didn't sound precious and exotic to the 12-year-olds of the time, but as slang-filled and modern as our own language sounds to us. And it certainly wasn't the language of Shakespeare.
I stand by my belief that, so long as historical periods are presented as esoteric instead of normal to the people who live in them, we will limit the way kids can relate to the past and recognize themselves in it.