A common topic of discussion among gamers reminiscing about their past is, "Which adventure modules have you run more than once?" I would contend that how one answers that question says a great deal about when one was introduced into the hobby and/or one's early experiences in it. I would hazard to guess that the majority of self-identified grognards would be able to list several modules they'd run more than one time. I myself could regale you with multiple tales of my having run, for example, David Cook's Dwellers of the Forbidden City, at least a half-dozen times, often with the same group of people. I expect this general pattern is true for many old school gamers, while it's something most younger players can't even imagine. How could I have possibly gotten multiple uses out of a single module whose page count didn't even break 30?
The answer is quite simple and it's this: old school modules were, by and large, descriptions of adventuring locales, whereas new school modules are plotted (loosely or tightly) adventure stories. That's not to say there aren't elements of plot in many old school modules. The aforementioned Dwellers of the Forbidden City includes a backstory about caravans being raided by mysterious enemies and the PCs are hired to go and find the source of the raiders and stop them. By the same token, many more strongly plot-driven modules, such as those in the Dragonlance series, include extensive descriptions of locales. There's a difference between the two types of modules, not merely in their focus -- which is important -- but also in their content. Dwellers never explicitly deals with the question of exactly who is behind the caravan raids. Presumably it's the yuan-ti, but the matter is left rather vague (someone can correct me if I'm mistaken on this point) and, at any rate, the backstory is there as an excuse to get the PCs to venture into the jungle and find the Forbidden City and its denizens. The real "plot" of the module is what happens to the PCs once they're there, not something the author has scripted in advance. That's patently not the case with the Dragonlance modules, whose entire purpose was to advance an epic story in which the player characters can take part but the general outlines of which were mapped out not by the referee but by the modules' authors.
If you're looking for a good signpost in determining where the old school ends and the new one begins, it's the shift in emphasis from locales to plots. It's not a hard and fast division; there are examples of both on either side of the line. Nevertheless, I think the expectation that a module "tell a story" rather than provide a location for a story of your own devising is a good indicator of where one's gaming sympathies lie. Since the late 80s at least, roleplaying has largely been defined as "freeform storytelling" and, given that, it's no surprise that adventure modules have been structured accordingly. Now, I don't actually think that definition is incorrect but it's misleading, because it doesn't take into account the increasingly heavy-handed role played by the manufacturers of modules, who've largely usurped the role of the referee in creating and maintaining his campaign world. This is why, though I love the folks at Paizo dearly, I've never been a fan of the whole "adventure path" concept that they've developed into a high art. I'd love to see them produce more location-based modules myself, but I expect they know their fanbase better than I do.
To some extent, I expect that the shift away from the location-based approach is a concession to the oft-repeated saw that gamers no longer have as much free time to make things up as they once did. I think this is hooey myself. More to the point, I do not favor the abandonment of modules entirely -- far from it! Rather, what I advocate is that modules should be made more, well, modular and that means providing lots of options and alternatives that a referee can then use to make its contents his own rather than someone else's. When I was a kid, I may have had lots of time to waste that I no longer have today. I can tell you, though, that I didn't spend hours and hours planning out my adventures, most of which were seat-of-the-pants affairs. What modules gave me was a structure -- map, room descriptions, game stats, etc. -- onto which could hang the story my friends and I created as I refereed their adventures. What I think has happened over the years is that, because "adventure module" has become so strongly associated with the notion of a pre-made adventure story, gamers now simply recoil at the notion of having to "make up the story yourself." They see it as too much of a chore, when, in point of fact, creating a plot/story is the easiest part of being a RPG referee, not the hardest.
I'd love to see some modern day easily reusable modules. Perhaps I need to write them myself.