I did a paper once that said:
Wizard = Shaman = Scientist
It had nothing to do with DnD, just the real world. I illustrated and made a comparision between the shamans of the past and the scientists of today.
More importantly, I explaned the relationship that the layman had with the tools and "scientific advances" that the scientist had created. Namely, how a person uneducated and unspecialized in the specific branch of science that created the advance dealt with the technology.
Example:
My parents do not know exactly how a television works. They know the basics: you plug the TV into the wall, "electricity" flows into the TV, and pictures fly through the air into the antenna. Then they can operate it by clicking the little buttons on a plastic box.
But if you asked them to build a TV from scratch, they'd have no idea where to start. (Besides, going to "TV making school". Or reading a book and making all of the necessary parts.)
My parents attended high school and have AA degrees. They could learn how to make TV's given the time and inclination.
This can be applied equally well to DnD.
Example:
Command word items. The noble layman has no idea how they work, but he knows how to activate the item. He pays money to the mage's boss because the mage makes a horrible salesman, and the mage then uses some of the money to make a better, more streamlined version of the command word item. The old item is discarded and trickles down to the poor.
In a world like DnD, where magic items last forever if protected from destruction, eventually you'd have mass production of everything. The magical version of the printing press, the railroad, the picture box. The magical version of a drill, jackhammer, crane, airplane.
Also, more and more people would turn from being "laymen" to apprentice specialist wizards. Schools and universities would sprout up everywhere. The populace would find it far more profitable to learn a couple levels of wizard than a couple levels of blacksmith, since the competition down the street is crafting battleaxes +1. "Wizardry for Dummies" books would sprout up everywhere.
Most people would not know how to cast Time Stop, but who cares? They can create Books of Copying, which are so much more profitable. Bladed Chariot of Plowing. Animated Brooms of Sweeping. Mighty Axe of Wood Chopping. The populace knows what 9th level spells can do, and is scared of them, but they trust that the government is keeping an eye out on those that would use them.
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In essence, any DnD world with magic becomes exactly like the current real world if some artifically-created, constantly-evil monster/god/apocolypse doesn't happen first.
Like the Netherese in FR or Sauron in ME or Darksun or what have you.
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Okay I have to tell this story now, since it directly relates to this discussion:
I came home from college one day and my Mom said, "Matt, what have you been doing on the computer?"
"What do you mean?"
"Have you been selling drugs or something?"
"What!?! What are you talking about, Mom?"
"Well, I think the government's found something on our computer...'
"Well let me see it."
She starts up the computer.
"See, I start up this program and it gives me this message..."
"What message?"
" 'This program has performed an illegal operation.' "