Greed = Tech
As far as I can tell, the driving goal behind the development of all technology has always been the desire for greater wealth.
Granted, it's not always spelled out that way, but boil it down far enough and it all comes out as greed.
We develop better agri-tech first of all to survive, but the best way to survive is to be so "food-wealthy" that you no longer need to worry about where the next meal is coming from.
We developed better arms and armor again primarily to survive, but also to be better able to keep our wealth from being stolen by raiders, and to be able to raid other nations to take their wealth.
Even in the modern day, technology's development is dictated by wealth. Internet R&D is funded predominantly by companies looking to exploit the web to generate wealth.
Transposing this to a D&D fantasy world is a rather interesting idea. In most D&D campaigns I've played in, the people in the world with the greatest wealth are nearly always adventurers.
Clearly then, technology is going to spring up to better enable adventuring. Following that assumption, we can infer that adventurers will fund the development of technology that will enable them to gain the largest return on their investment.
It's unlikely that anyone would fund a program to develop better ooze-fighting equipment, as oozes usually have no wealth to plunder. Likewise, it's doubtful that kobold-killing technology will see much improvement, since there's normally little wealth to gain in such endeavors.
So then, what can adventurers encounter that will earn them big enough rewards to make it worthwhile to pursue technological developments to improve their efforts?
Again, assuming a fairly "standard" D&D world, the first thing that springs to mind is Dragons!
Here are these big, dangerous lizards that normally have ridiculous amounts of wealth. Any adventurer could become filthy rich by specializing in dragon-hunting.
To that end, it's likely that many adventurers would do so. They are in turn going to equip themselves with the best dragon-fighting gear they can get.
This will add wealth to those vendors that can supply such gear, which will inturn encourage them to develop more/better pieces of such technology.
The first merchant that can produce such items as dragon-bane weapons, or armor that provides protection against dragon-breath will be in a very competitive market position.
Such a merchant would have a competitive edge similar to that held by Microsoft. Meaning once people discover the effectiveness of this vendor's gear, every other vendor who wants to gain more wealth will be forced to produce their own version of such gear, or become irrelavent, and lose market share.
This has a real world analogy as well. As soon as the first smith in a culture learned to work with iron, bronze, brass, etc were things of the past. Those merchants who insisted on selling such obselete gear soon were outpaced by the new technology, and either learned the new techniques, or lost their business, and wealth to the new craftsmen.
But of course, dragons aren't really the largest sources of wealth available for plunder. The real money will be in conquering nations. This of course will lead to technological developemtns in seige technology, and possibly even the creation of contructs, ie. warforged.
Follow this logic to its ridiculous end, and now we find that adventurers seeking even greater wealth, will soon need technology developed to hunt an even more dangerous game - gods.
Any enchanter that can come up with a god-killing device will find a source of immense wealth if he can readily provide such technology to adventurers. And history shows us that if the wealth is there to fund research, someone will develop a technology to meet the demand.
As an aside, one funny side effect of this economic demand for better tech is the inevitable creation of fakes and scams. Sure as someone will buy snake-oil and hair-tonic, or send money to Nigerian email scammers, someone in a fantasy world is going to try passing swords doctored with a false magic aura as "genuine-bona fide dragon slaying blades!"
Additoinally support services are going to experience a boom as well. Medical treatment for burns will become more advanced, if an area is plagued with red dragons, and sees plenty of burn victims, you can bet that someone will invent a healing salve that specifically targets such injuries.
The real irony is that if you want to follow the ultimate progression from dragon, to nation, to gods, there's another logical branch, and that's adventurer!
The first adventuring party that kills a god will achieve massive fame and noteriety. Surely some other would be god hunter will realize that the shortcut to wealth would be to just kill other adventurers and take their treasure.
If they conciously pursue that path toward wealth, it would only be sensible that they would fund development of new technologies soley designed for disabling adventurers.
Of course, such devices work both ways, and if they became widespread enough could eventually be used against their creators.
If such technology followed real-world patterns, such weapons would become increasingly easy to use. Once you go down that path, it's easy to picture a world where the technology makers realize that they have developed items that will allow them to disable and loot the adventurers themselves. At that point, you have a world where the commoner and his technology become the driving force in the world.
This kind of scenario could lead to a world where technology is eventually used to supplant magic entirely, rendering the fantasy world a stagnant mudane realm.
Of course at the point, someone will start developing a new technology to bring magic back into the world.
Just to accumulate wealth.
Just in case Ranger Wickett likes this post - Tournaments, Fairs & Taverns would be a very welcome addition to my collection.