The teacher in me wants to write a rubric for grading adventure modules. I'd rank them in each of these categories:
Design
The adventure is a moderate challenge for characters of the stated level. TPKs are possible through patently foolish or rash decisions by the party as a whole, and individual recklessness or carelessness could spell death for a single party member. The party's objectives may evolve over the course of the adventure, but at any given point, their goals are clear. Each puzzle solved or monster defeated moves the story forward, and the adventure unfolds with a growing sense of wonder, dread, and excitement. The adventure is rich with moments of revelation in which the PCs realize connections between seemingly unrelated elements, achieve a clearer understanding of the problem, or see past events in a different light. The resolution of the adventure is a moment of high drama and a logical conclusion to the story. The DM is given an opportunity to continue the story or make it a self-contained adventure. Players exit the adventure with a satisfying reward of treasure, items, XP, and new insights about the campaign world, eager to continue.
Writing
The adventure uses language that is clear and concise. The read-aloud descriptions are vivid and evocative without being too wordy. Notes to the DM are unambiguously worded and organized to allow a reader to pull out key information quickly. Seldom-used rules from the SRD are included to remove the need to look them up in-game.
Production
The editing is thorough. No typos. Text formatting is consistent throughout. The layout has a clear information hierarchy, with essential information easiest to find, not unlike a newspaper. Stats for monsters and items are easy to find.
Visuals
The illustrations are of professional quality, whet the DM's appetite for running the module, and actually help clarify the text. The maps are easy to read and cover the action of the adventure comprehensively.