There are really only two paths for a "real D&D society," due to the way people behave and believe. It comes down to a power struggle between practitioners of magic versus those of "the faith," and I'm inclined to believe the later would gain popular support. Still, here they are:
In a world where Wizards rule, there are certainly no castles and knights, and even armor would be unlikely to have progressed much past chain due to role arcane warriors would play on the battlefield. And if you think wizards would be relegated to some "advisory" role or sequestered scholars searching for lore in a library, then you have no clue what people really do with power. Think about it; ancient people came up with all kinds of myths and legends to explain things like what makes lightning during a storm -- myths and legends that pretty much always involved angry gods or pissing off ancestors or something. Imagine those people confronted with groups of other people who can do those very things, no myths or legends needed. Wizards would be living gods, and you'd probably have entire cultures that worshiped and fearedthem as such. If spellcasting is something that can be taught, you'd probably end up with a world that resembles Thay.
In a world where Clerics rule, things would look like a combination of the Thay and real history. Religion has been a dividing force throughout all of human history, and no matter how much the source material might talk about things like peach, harmony, and forgiveness, the actual people in charge of the religions and/or societies pretty much always used it as a giant baseball bat of fear to keep the population meek and controlled. If you look at, say, the Christianity religion in context to the afterlife, you'll see a book that often resorts to fear as a means of encouragement. Passages like "If anyone’s name was not found written in the book of life, he was thrown into the lake of fire" don't exactly do much more than scare this crap out of people, especially with that huge painting of naked people being clawed by demons right next to the friendly priest. That stuff must have been scary for a population that knew death could come at any minute, and without warning, without any expectation to live much beyond 30. Imagine that kind of desire to influence a population that's also backed with the actual use of divine magic. It would be like living in a massive abusive relationship, where one minute your being healed of your syphilis, and the next your watching people burn at the state for practicing "forbidden magic."
Those are basically the only two possible "real D&D" societies I can see happening, outside of the common man violently overthrowing and killing all arcane and divine spellcasters every few hundred years.