This has probably been said more than once in this thread, but thinking about the real Middle Ages, I don't think they're a time period I would want to have lived in, plus I'm not sure how appealing it would be to spend time in a simulated, non-romanticized version of it. I mean, consider the features that would have to be advertised:
Play all of our game's ONE playable races!
Play as a peasant man: spend all day plowing fields, milking cows, shoveling out horse stalls, harvesting crops, driving wolves away from the livestock and then have the nobility show up and take half of it from you in exchange for not chopping your head off.
Play as a peasant woman: Spend all day cooking, weaving, cleaning, washing clothes, have almost no human rights and get boned by any horny nobleman who sees you bend over in the garden.
Experience the thrill of having no opportunity for a better life as you climb from levels 1-50, from a dirty, put-upon peasant to a still-dirty, still-put-upon peasant.
Catch a myriad of diseases, such as gout, shingles, rickets, tuberculosis, wet lung croup, influenza, fever, septic infections and the black plague. Employ dozens of folk remedies to heal yourself, like herbs, leeches, unsanitary amputations, all of which will never work (well maybe the amputations, but they'll almost always lead to another disease due to rusty blades and filthy living conditions).
Engage in thrilling, one-move combat where you die from a single blow from an enemy.