Here's why I like those modules so much:
- They had interesting plots and backstories that had an effect on the adventure. When you found the weird artifact, it's backstory provided the framework for the adventure.
- Cool, interesting, high magic stuff played a fun role in the adventure. The elemental air "subway system" drove home the power and accomplishments of the ancient civilization whose ruins you set out to explore. It also gave you a sense that you were going somewhere distant, and maybe getting back would be hard.
- The adventures had an epic feel without going over the top. You helped defend castles, spoiled the machinations of the most potent force for evil in the Grand Duchy, and saved an entire order of monks. It was never just a matter of beating up some orcs.
- The art was evocative. It depicted classic D&D stuff in a recognizable manner, yet still retained its own distinct feel.
- The maps were clear, easy to read, yet drawn in an evocative way. They looked like maps someone drew, rather than maps printed by the local tourism board.
(BTW, Axe Initiative Games has posted their writing guidelines for those of the would-be or currently-are writing persuasions.)