Background, Setting, Antagonist(s), Mystery (including surpises and new "stuff"), Versitility, and the Rational which ties it all together.
Some classic examples, off the top of my head:
The Steading of the Hill Giant Chief. I've used that adventure dozens of times over the years as a setting for anything from rescue missions, escape scenarios, even a "random encounter" where the PC's encountered a band of hill giants and tracked them back to their lair.
Keep on the Borderlands, for similar reasons. Tons of NPC's between the Keep and Caves and a wide variety of things that can happen, including murder and political intrigue.
I think one of the important keys to a good adventure module is that you should be able to remove the author's "story" and still make full use of the adventure.