I think some people are arguing that you _can't_ have a medieval-type society given the prevalence of magic in standard D&D, because some wizards & clerics are bound to use their magical powers in such a way as to change the nature of the society, creating mass saniutation, industrial production et al. I think this argument is completely wrong, as much as it would be wrong to say that a medieval society is the _only_ way to play D&D in. Why is it wrong? Some reasons:
1. Medieval-type society is not in fact a natural step along a continuum from primitive to modern. Rather, in our world it came about because of the _destruction_ of a much more civilised society: it is a post-apocalypse phenomenon.
2.Medieval society was one in which almost all of the effort of those in power - the rulers - went towards military power: knights, castles, heavy warhorses. Compare this with the Roman empire, which for most of its history had tiny armies and vast public works including functioning sewers, clean water delivery systems, bath houses etc.
3.The result of this is that the level of medieval battlefield/military technology is in fact MUCH HIGHER than the general technological level, and the D&D spells reflect this extremely well. If the society you're reflecting in your game is both civilised and reasonably high-magic, I reckon there should be as many non-military uses for magic as military ones. But for the medieval paradigm, it's the ability to kill things that matters.
As an example, my campaign has run for about 400 years game time, 16 years real-time. In that time the general technology level of the primary world, Ea, has progressed roughly from 1100AD to 1500AD, although a looming dark age may put an end to the nascent renaissance. Extrapolating Ea's history back in time from before the start of the campaign, there's roughly a 1:1 gameworld year/historical year correlation in tech level - 1000 years ago tech level was ca 100 AD, 2000 years ago 900 BC, and so on. In the 16 years/400 game-years I've run Ea, I've never seen any PC make a significant positive technological impact on the world - the only major impact was in fact a negative one, the PC god Thrin, god of knightly combat, altered the world's physical-factor to prevent the use of gunpowder and similar chemical technology, thus preserving the preeminence of the armoured knights he loves. And that's a Lawful Good PC! The less good-inclined ones are more inclined to just go around trashing everything. Hardly out of character for the medieval mindset, either.
Now, it IS possible to use magic to start an industrial revolution, given a kindly GM. But this is a function of mindset, not the nature of magic. Mark Twain covered this very well over a hundred years ago when he wrote "A Connecticut Yankee in King Arthur's Court".