Terseness: The old modules could pack A LOT of gaming into just a few pages. Take B2: Keep on the Borderlands. This module easily packs 32+ hours of gaming into as many pages. The module may seem thematically simple, but it had alot of different monsters in it, a variaty of traps, and treasures hidden in almost every way imaginable. It chiefly suffers from a poorly designed map IMO, but remapped with a more classic approach and with a decent DM setting up the struggle between the keep's defenders and the insidious evil cult of chaos I dare say it would stand up to much of what is published today.
That last point is critical to understanding why the old modules are so popular. Alot of stuff necessary for a good module has been left out of virtually all the old classics. It's entirely up to the DM to add the missing material and flesh the adventure out. It's precisely because we've seen them run in thier proper glory that we think well of the old modules.
Purity: Until recently, the quality of the old modules as pure dungeon crawls had not been equaled. It isn't until you get modules like RttToEE and Sunless Citadel and HoNS that you really start to see the art of the dungeon matching the early dungeons in sheer quality of design. There is something very compelling about classic dungeon crawling that seemed to have been lost for a long period in D&D. I can tell almost immediately whether a player learned his dungeon crawling skills in the old school dungeons. The old school players have a lot more faith in the power of 10' poles, long lengths of rope, bottles of whiskey, flasks of oil, torches, iron spikes, chalk and the like than modern players. The old school players are far more methodical in play. Newer players generally have bad 'dungeon hygene', are inappropriately incautious opening doors, and tend to think searching a room thuroughly involves only throwing a d20.
Map Quality: I complained about B2's low quality map design, but in many cases the map design of the early modules have never been equaled. C1 'Hidden Shrine of Tamoachan' is brilliant. S1 is a masterpeice of sparse design. Very little is wasted and though the module is actually highly linear, it never feels linear. U1 is awesome in conveying story through location, and until 'Sunless Citadel' was probably the definitive introductory D&D module. The real classics though are I3, I4, I5, and I6. Simply put, the map design for I6: Ravenloft has never been equalled. In terms of use of space, Castle Ravenloft is the best designed dungeon in the history of the game. The Pyramid in I3 is a close second. There are some equally well done and equally inventive 3D maps in the Dragon Lance modules, but they lack the terseness of I3 and I6. They have far too much blank and redundant space.
Scope: The early modules really felt like massive adventures. When you were done with them, you felt you'd accomplished alot. Your typical modern 16 page or 32 page adventure feels like a television episode in an action adventure series. It's by comparison breezier, quicker, and more episodic than an old school module. The 'Tomb of Horrors' only has 32 small locations, but when you first journey into its depths it seems frightenly enormous.