That's easy: the maps.
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DL8 - Map to the Tower of the High Clerist: A castle that is nearly 800 feet tall, with level after level after level of floor map designs and layouts for you to fill with whatever your heart desires. Still the largest published Castle Map of all time, AFAIK. If you wanted to run the whole damn dungeon as one campaign - 1st to 20th a la WLD, you easily do so. As the lair of your BBEG and the last dungeon crawl of your campaign? This is >>Da Shizznit<<. I have often re-used sections of this map in other campaigns. I don't think my players ever recognized it either. It's a MASSIVE castle. The module itself does not even pretend to detail less than a few dozen areas. There are Hundreds and Hundreds of area in the thing. It's MONSTROUSLY HUGE, okay?
You inspired me to go down and pull out my copy of DL8. I have to admit that I probably - make that definitely - haven't cracked it open since I bought it in 1985, so it was crisp as if brand new. (We were playing the modules at the time, so I didn't read them; we got through DL6, IIRC, before the group fell apart, and I never found time to go back and really read through.)
You are NOT KIDDING about the map of the High Clerist's Tower. About 22"x32", 16 levels plus the 3 levels of the Knight's Spur, highly detailed - PLUS an overhead battlemap of the whole place on the flip side. Incredible. Plus, on flipping through the module, it has a full page dedicated to sheet music for hymn of the Solamnic Knights. Talk about detail and immersion in the campaign world.
Oh, and the cover price? $6. (That's about $12 in 2010 dollars.)
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The "shared experience" observation is spot on, but I think some of the blame for poor 2E product needs to be laid at the feet of TSR's business practices. As a consumer, I got more and more frustrated at products with large fonts, huge white margins, poor maps, and ended up not buying a lot of 2E past '94 or so. (Didn't help that I went back to college and had no spare $$ for such things.)
It seems to me that as TSR's financial situation got more and more dire, they tried to cut corners and save money with such tactics, and it ended up backfiring on them in the end. Eric noted Return to the Tomb of Horrors and Gates of Firestorm Peak as good 2E mods; both of those were produced after the WotC purchase, IIRC. In my opinion, a solid, stable company and manangement supporting the creative team made for a higher quality product.