What makes a classic module?
If I feel it's a classic, it is. Heh.
More seriously, probably the most important criteria are that it is both well known and popular. It would be hard to find better known modules than the Keep on the Borderlands, Against the Giants and Tomb of Horrors. There is an important shared experience with those modules, common to most older D&D players.
After that, there is a necessity to examine what the module brought to the D&D game. S1 is the archetypal "Killer Dungeon". GDQ, apart from being the first AD&D modules, also gave us an epic quest, the concept of the underdark, the drow, and the idea of actively demons interfering in events, as well as many other factors.
T1 gives us a campaign setting (with attached adventure) that could evolve.
As I said before S2 is a good example of "Monsters, Traps, Tricks and Treasure".
B2 is
the fun introduction to D&D. The module also looks outwards, allowing the DM to build up the entire setting even more. There are parallels here to T1, although B2 handles the adventure section better than T1, and T1 handles the PC's base (slightly) better than B2.
X1, which I really should list in the classic section, is important in how it handles the wilderness: it's one of the few modules that is almost entirely wilderness, and the exploration of the wilderness at that. There's something extremely important implied by that blank map of the Isle of Dread with all the hexes for the players to fill in. Exploring the Unknown! I do think there should be more adventures like this one.
Apart from that, a classic module requires a certain amount of thematic cohesion. The theme in the GDQ modules is extremely strong - defeat the giants, then descend into the depths (although this isn't apparent at the beginning of the quest!) It's one reason I'm not so sure about the A series of modules as a whole: the grand theme of the slavers is extremely strong, but there are encounters in some of the modules (like the Aspis in A1) which I don't think quite fit in - a legacy of the tournament origins of these modules. A4, however, is definitely a classic.
U1, I3 and I6 bring in a new style of play: more role-playing or story-orientated than what came before. As I mentioned, I think that overall the U series is too slight on gaming material. To a large extent DL1 is a stage further in the progression from these modules - it suffers from being one of the Dragonlance modules. It is one of my favourite modules - it is a "near miss" for inclusion in the list of classics. In fact, DL1 suffers from a confusion as to how to get the PCs into the story, and this structural flaw is severe. It's great once the players are into the main story, but the DM can have a lot of problems getting them there. I've run it three times, so I've had a lot of experience with it!
There is a type of module that I regard as a "tournament" style of module: it has a quest, and then various monsters, traps and tricks to challenge the players. There are plenty of these sorts of modules, and to some extent they blend into one another. S2, S4, C1, C2, A1, A2 and A3 all are of this sort. So are other modules I've listed, and a few I haven't listed, but I do think that these are most noticiably of this sort. Of course, these module all originated as tournament modules, but they are more homogeneous in approach than, say, G1 is. You could probably list them all as "classics", but I prefer to only list those that I consider exceptional for one reason or another. I list A1-4 due to the great theme of the Slavers, and S2 because I consider it to be exceptional in its presentation.
The actual presentation of the module is important as well. When I think of the formatting and artwork of I3, B2 and S2, I am near my ideal of module presentation: easy for the DM to assimilate and use in his or her game. This is one reason I'm not so fond of L1: I don't like the presentation of the material very much.
N1 suffers (for me) in its presentation as well. I don't like how the text looks on the page, and I abhor Holloway's art.
In any case, that's how you discover which modules are classic or not. Simple, isn't it?
Cheers!