The Fourth Year: 1981
S2: Expedition to the Barrier Peaks
L1: The Secret of Bone Hill
I1: Dwellers of the Forbidden City
U1: The Sinister Secret of Saltmarsh
A2: Secret of the Slavers Stockade
A3: Assault on the Aerie of the Slave Lords
A4: In the Dungeons of the Slave Lords
X1: The Isle of Dread
X2: Castle Amber
B3: Palace of the Silver Princess
One of the most interesting adventures of 1981 was the first release of the Expert D&D series of module: The Isle of Dread. This was the first module to be made up of adventuring in the outdoors wilderness (as opposed to the underground wilderness of D1-3). X1 was an exploration module where the PCs ended up on Skull Island with a hexgrid map that they had to fill in as they went along. I don't know of many other adventures of this type - if any! Mostly the wilderness is not mapped by the players. Added to this was the Lost World/Skull Island motif, and the adventure must be considered a classic, though it still required a good DM to fill in the additional details (and to make the Cthulhu-like Kopru significantly scary).
The other Expert-series module, X2, also owed its debt to written works, in these cases the works of Clark Ashton Smith. I am not sufficiently familiar with either the adventure or the books to comment further, however.
The problems that beset the original version of B3 have been detailed
elsewhere (and
here), again, I'm not sufficiently familiar with the revised adventure to comment much upon it. As I recall, it was turned into an entertaining quest adventure, which set the template for the more structured modules that would arrive from the Basic line.
Returning to AD&D, S2: Expedition to the Barrier Peaks is one of the great classics. There is no big villain (as there was for the Giants, Drow and Tomb), but instead just a strange metal dungeon filled with very strange devices and monsters. Kill monsters and take their stuff - certainly - but with a fantastic and extremely challenging environment to do it in.
A2 and A3 followed the pattern of A1 and C1: challenging encounters for tournaments. Little here that we hadn't seen before. A4, however, is something special as the PCs had to escape from the dungeons without any equipment!
I1, Dwellers of the Forbidden City, was the first installment on the "Intermediate" series of modules for mid-level characters. It was another tournament module, although fleshed out more than others of the time. It is primarily interesting for introducing the Aboleth and Yuan-Ti, and for being set in an exotic tropical setting. This is another of the classic AD&D modules.
Finally we come to L1 and U1, two adventures for beginning characters.
U1, The Sinister Secret of Saltmarsh, was the first adventure of the UK branch of TSR. It is well regarded, and is actually quite unusual. Containing two linked adventures, it takes remarkably little time to play through. The first part deals with a "haunted" house that actually turns out to be the home of smugglers; the second part deals with a raid on their ship which finds more secrets that would be dealt with in the next installment. It also suggested a greater involvement of the home base than was normal, although it left much of the detailing of the town up to the DM.
L1, The Secret of Bone Hill by Lenard Lakofka, followed the pattern of T1 by detailing the home base of the characters in some detail. Where it varied from that pattern is by also giving a lot of detail about the countryside and the adventure opportunities there. When I consider Lenard Lakofka's articles in Dragon magazine, I see someone who wanted to add more detail to D&D than it needed. That trait is definitely in L1!
I find the adventure to be confused in layout, though containing much good material. Similar to T1, the PCs begin in the town trying to discover details about adventure opportunities - a striking contrast to the "here's the dungeon and go get it!" of other modules. For reasons that Erik touches on above, I think that's why I've never warmed to L1 and T1 as much as others.
Cheers!