If I'm standing in FLGS and perusing an adventure, the criteria that will make me purchase the book are laughingly superficial:
- Thematic art, especially of NPC's. They are the most important part of the adventure for me.
- Clear maps at all scales. The adventure locale should have one of the best pieces of art in the game, with all key locations marked. Give me good maps, and I can hold the adventure together.
- More than one inspired location or encounter. Flicking through, you can usually spot the encounters that have received the lion's share of the writer(s)' attention. What I want is a "Huh, that's cool" moment.
- An actual finale, and some indication that the writer(s) have considered different paths to it.
With those signals, I can usually have fun with an adventure. Tick those boxes and you might have a sale. The actual qualities of the adventure, its coherence, its accomodation of the PC's actions, its pacing, and its surprises and its thrills, are almost impossible to judge from a quick read-through, so I've stopped trying.