My friend Darryl C's dad, Big Darryl, used to say I went to college and majored in D&D. I'd laugh because he was kind of right. I was a history major with a concentration on the European middle ages and a medieval and renaissance studies minor. Even for my non-concentration courses I kind of cheated the system and took stuff that enhanced my D&D verisimilitude skills, like ancient history courses, anthropology, ancient philosophy and feudal Japan. I didn't actually play a lot of D&D at the time, although for part of the time when I was in college I regularly played in a 2nd edition AD&D game on Sundays.
My time in college did give me a great appreciation for the real medieval period though (and the ancient world) and it made me a little dissatisfied with “standard” D&D campaigns. I started my long period of trying to make my D&D game settings more realistic with regard to customs and cultures and laws. My style of campaign setting has annoyed some players in the past, and my wife has occasionally remarked that my D&D games come with homework assignments for the players. Having said that, my games have pretty much always been popular with a variety of players whether they were short mini-series style campaigns or one-shots or planned as long-running campaign settings. I have run them all, using the TSR 2nd edition HR series, GURPS setting books and my own library as references or just based on my own fairly extensive knowledge.
What I haven't been able to do is run a “real” medieval/feudal setting. I think that it's because that is the default fantasy gaming setting and the default gets too much wrong. When the culture is very different players approach it differently than they do the pseudo-medieval regular D&D setting. Oriental Adventures means that the players have to make the effort to pretend to be Asian (usually feudal Japanese). My Celtic games or my Viking games or my Roman games get the same kind of effort, if not at the same level. You get the same thing with other genres, Star Trek or Star Wars games make the players think and act like characters from their respective universes for instance. For some reason though, it just does not play with a Feudal European setting and I have to blame the tropes of D&D for that.
In a standard D&D world we have all of the expected medievalisms, knights and castles and what-not. But the players play their characters as, more or less, modern people; even when they are a different race. They don't try to alter their mind set for the campaign setting. The player's mind set for the faux-medieval world of D&D is more like the wild west than the middle ages (which is odd when you think about it because wild west games have never been all that successful) or an Indiana Jones movie. There are no social, political or legal ties for most characters, monsters are commonplace rather than fantastic and religion has relatively little significance in everyday life for the average adventurer even when it is ostensibly their job. Yes, I am looking at you Cleric and Druid. Very often standard D&D games remind me of the movie “A Knight's Tale” with Magic and monsters added. This is probably why most D&D games are set on a frontier of civilization I guess.
I don't really know how to fix this, or even if it's a problem for anyone but me. I have occasionally had my players, singly or in groups, ask me to run a “regular” D&D game next time around. I don't hate it either. I have had a great deal of enjoyment from playing in and DM-ing for “standard” or “regular” D&D games. I just think that maybe it could be fun to play D&D in an actual fantasy medieval setting.
The exception to this rule is, almost exclusively, Pendragon; which is of course an entirely different game from D&D, I just felt I needed to acknowledge that it was possible to get modern Americans to think more medievally. I have considered using the Pendragon setting for a D&D campaign, but I go back and forth on that. On the one hand it would very likely work, making the players fall into the proper mind-set and all, but, on the other hand, there is a lot of work to do there to convert and the Arthurian setting isn't exactly what I am looking for.