Two reasons why many 1st ed. adventures are good (aside from nostalgia, Erol Otus artwork, and other subjective stuff):
(1) They're generally unbalanced and too deadly by 3rd edition standards.

Which is refreshing, at least to me.
(2) More importantly... they have a lot of sheer RAW MATERIAL in them. By which I mean, the best 1st edition adventures have a LOT of dungeon (and sometimes wilderness) maps, a LOT of locations, a LOT of statistics & monsters & treasures. Pure and simple. They *don't* waste a lot of time on role-playing suggestions of flavor text or anything like that.
Now, before people accuse me of being a hack-and-slasher... I love the role-playing aspect of RPGs. I love non-combat stuff. Nothing pisses me off more than shoddily constructed campaign worlds or bad ROLE-playing.
But if you have to adapt an old adventure to a different version of D&D, or to a new campaign world, the stuff which is easiest to adapt is the raw data. The raw maps and monsters. The nice thing about many of the 1st edition adventures is the very fact that they're so generic... they have a lot of quirky little details, but they're easy to plug-and-play into my campaign world, which is nothing like Greyhawk at all.
For a comparison of a "good and simple" first edition module with a "bad and overwritten" second edition module... compare the original KEEP ON THE BORDERLANDS to RETURN TO THE KEEP ON THE BORDERLANDS. RETURN has a few interesting nice touches, sure, but basically, it converts a really simple, short-and-sweet adventure into an unwieldy module with tiny print and WAY more descriptive text than is necessary for the slim amount of actual locations, monsters, threats, etc.
In short -- the best 1st ed. modules are good precisely because they're so "generic" that they can easily be reused and adapted into many different forms. There's no extraneous information. I've been converting 1st edition modules to 3.5 for awhile now, adding my own character-motivation and campaign-setting, and it works very well.
Jason
P.S. On the other hand, for an example of old modules that are "generic" but totally sucky and un-useable even by these standards, I'd say that the old ARDUIN modules (like "Death Heart") are illogical and capricious to the point of absurdity.