I mostly agree with Celebrim. In my mind, who advanced a society is depends on the amount of energy they can produce and how efficiently it can be used. In my mind, magic can produce a <i>lot</i> of energy, especially the way it is treated in most "high fantasy" game . . . where high fantasy generally means that to have an experienced, effective character that person is carrying around a small mystical arsenal. PLUS, in most D&D games -- the ones that sort of willy-nilly use things from all the core books and a double dozen supplements -- there's all sorts of things that once you put them in could concievably lead to massive changes. Like Decanters of Endless Water and Teleport Circles for just two of literally <i>hundreds</i> of examples.
To create a fantasy world where this is <i>not</i> the case, you'll have to get pretty radical. In my first post, here, one of the ways I proposed this is to have being a magician (be it divine or arcane) have mystical and psychological reasons for not wanting to see the average people advance. Perhaps becoming a powerful magician makes the magician indifferent to the suffering of others, for instance, as a side effect of the forces they play with.
Other reasons could be metaphysical. Perhaps there is only a limited amount of magical energy produced by the universe and too many magic widgets will start to deplete the magic faster than the world can rejuvenate it. This would be a compelling reason for magicians to limit the number of magic items they create -- after all, if too many magic swords in the world would eliminate your ability to cast your most powerful magic spells wouldn't you think twice before making one? This limitation could as easily apply to the gods, so clerics would also have an eye to insuring that too much magic wasn't being drained away by the presence of magic items.
Other reasons could be . . . well, you can just get far out. Take what I call Faerie World (a campaign I fully intend to run some day): the world is populated by immortal, absurdly powerful creatures who evaluate things totally differently than we do. Since they're more or less incapable of feeling worry, pain, hunger and fatigue (at least as we mortals do), the things that drive us to technological and magical prowess would mean nothing to them. Who cares if the crops come in? They don't. They'll continue their wild feasts one way or the other. So they could well burn all the fields for reasons unknown and perhaps incomprehensible to mortals. The same can be said to be true, probably, of any world that has immoral beings that control the world, and even celestials could have a radically different of what "good" means than even the best human mortals. I mean, what if the celestials got together one day and decided that towns with more than five thousand people were fundamentally evil? I know some people who would argue, roughly, that cities are evil, too, so it would even make some <i>sense</i> to some people.
To ramble on a bit, consider this: a bunch of celestials get together and decide that technology is too neutral for them. People who use technology to feed themselves, to heal their wounds, etc., no longer have as much motivation to patronize the local good clerics as before, so technology leads towards evil. They arbitrarily set a level of technology that can't be exceeded before very unsympathetic guys with flaming swords start reminding people "angel of death" is a legitimate term. The same could be true of any system, any system at all, that introduces too much efficient energy use in a society (decanters of endless water being used to power ships in any numbers, organized teleport circle clusters, etc.) -- it would be forbidden <I>by the good guys</i>. They'd tell people, y'know, "Seek not more than you need to make you happy." Lots of people would buy it if angels came down and reminded them of the truth of it, from time to time -- it would swiftly become part of the culture. Especially after the purges and re-education.
Fiends might also benefit from this system -- they'd always have something to offer to people. "Want to escape the tyranny of this world? I've got this recipe for something I like to call <I>gunpowder</i>." That, in the end, the gunpowder will be surpressed by the celestials might not be seen as a disadvantage! The fiends might shrug and say, "Great, now I can use that same trick <i>again.</i> Thank you, Mr. Angel."
But give the amount of freedom most D&D games seem to have, there'd be deuced little to stop what Celebrim talked about from happening.
