I agree that with most of the recent posters who have said that the portability of these "modules" is one of hte main reasons for their "classic" status. I use published adventures fairly often and usually modify them somewhat to fit my campaign. When I am planning my campaign, I usually look through my adventures to see if any of them would fit into my campaign concept. Usually, the easiest adventures to plug in are those story-light classics that provide some good locations, maps, NPC's, etc., maybe even a good BBEG to start working into the campaign before the PC's get to the adventure. If an adventure is too story-heavy, it is too difficult to work in.
As an example, take Necromancer Games' "Crucible of Freya." This adventure has a very specific plot. Bad guys come to town and steal a cup. PC's have to chase them to their keep and get it back. When using this adventure, I had to scrap the entire story and use its BBEG in a totally new way. Thankfully, NG seperated the story text from the location text and put the story in the first half of the adventure and the locations and NPC's and maps in the second half of the book. This made scrapping the story much easier.
By contrast, I followed that adventure up with the old T1: "Village of Hommlet" (which seems to be a pretty popular adventure right now). There was little plot or story in that original, so I just gave Lareth motives that fit my cmapaign. I could use almost everything in the book and it still fit, rather than only using half of the material, like I had to do for Crucible.
It is, IMO, much more difficult to port any of the Dragonlance adventures or even most of the Forgotten Realms adventures since they are so tied into the campaign world. Even most of the published Eberron adventures seem to have something in them unique to that world, like dragonmarks or whatnot that make them seem artificial if you try to port them to a generic or homebrew world, or even Greyhawk (which most would say is THE generic world).
Some adventures that I have ported into the campaigns I have run since 3E came out:
White Plume Mountain and Ex Keraptis Cum Amore (from Dungeon)
Falcon trilogy
Lost Tombs trilogy (I know, 2E, but still pretty light on story for the most part)
countless adventures from old Dungeon magazines (many are very story light and portable)
Village of Hommlet
Isle of Dread (also used Portents of Dread from Dungeon)
Treasure Hunt
Destiny of Kings
The Forest Oracle
U1-3 Saltmarsh trilogy
Ghost Tower of Inverness
The adventures I have used from 3.x are:
Sunless Citadel
Forge of Fury (VERY modified)
Standing Stone
Heart of Nightfang Spire
Bastion of Broken Souls
Against the Barrow King
Freeport Trilogy
Crucible of Freya
Tomb of Abysthor
countless adventures from newer Dungeon magazines
Out of the two lists, I am sure that not all of them could be considered "classics" or even great adventures, but I think there are more of those on the first list. How many of the adventures on the second list will become "classics" remains to be seen.
I would love to see comments from one or more of those writers who designed the "classics" regarding what, if any, design principles or philosophies were used. Many of the newer adventures come across, to me anyway, as feeling formulaic or contrived. There is too much of an effort to include x number of CR y encounters, etc. That is one of the things missing from older adventures that makes them better. Newer adventures spend too much time either trying to replicate older ones or escape from them.
Well, just my $0.02 contribution to a thread I have enjoyed reading.
DM