In one corner we have the 'classic' style. It is basically one or more maps, a lot of tbk inhabitants and sometimes some memorable treasure. Hook, plot, and context have to be provided for by the DM.
If run without preparation by the DM, this adventures are basic examples of the Enter-the-room-kill-the-monster-take-the-stuff style. If run by a creative DM investing time and work for the preparation, adaption, and personalisation of the adventure, there are no limits of what level of play can be reached.
In the other corner we have the 'railroad' style. Taken to its extremes, it is little more than a game-book, where the story is interspersed with decision points.
If run without preparation by the DM, players will experience an illusion of a story where their characters are the actors. As long as the illusion persists, i.e. the players believe that they actually control their characters and, thus, the story, they may experience a great adventure, but as soon as the illusion breaks down...
Oh wait! We have a third corner over there! The string-of-pearls style! It tries to combine the advantages of both its competitors. The DL-series is, as far as I know, the first example of this style. Each single adventure is a lot like in the classic style: the players enjoy a lot of freedom, the actual plot is not overly enforced. But to proceed from one advenuter (pearl) to the next you have to maneuver through the connecting string.
When I lived through the series as a player, our DM tried to give us free reign. After destroying Kisanth in Xak Tsaroth, he let us go wherever we wanted to.
We got hopelessly distracted (left the string) and nearly didn't 'find' the next adventure.
The string-of-pearls style with pearls of freedom connected into a compelling plot is my favourite style of adventures. This model scales very well; you can actually use it to construct a single adventure made up from several parts. A good example for this would be the first adventure in the Age of Worms AP.
The bad news is that I have yet to see such an adventure or campaign actively using this structure. The connection points or interfaces are in most cases not well defined and described. With 'interface' I mean all the information necessary to proceed from one pearl to the next. What do the characters have to achieve/get/know in order to proceed? What triggers (for plot driven adventures) can trigger the next 'pearl'?
This information is essential but usually well hidden in the text or sometimes even not clearly expressed at all: "The heroes can use the Key of Utmost Importance to open the door... ? Uh, but my players don't have the key. Damn, I don't even remember where and when this KUI was to be found!

Uh guys, you can't open this door... there is actually no way to open it at all

"
Ah well, enough rumblings for now. Hungry children waiting for their dinner
Huldvoll
---Jan van Leyden