Fantastic post over at the Underdark Gazette, reviewing the module Pyramid of the Dragon.
The module sounds top-notch, and the review really highlights some key elements of top quality adventure design.
The first is "the temporal element". The adventure should have living breathing plot elements, and a cast of characters that operate on their own schedule. They should not just be frozen set-piece waiting for the PCs to trounce over. The monsters should have their own agendas and schedules.
The second element is "monster on monster action". It basically grows out of the first element. Monsters have agendas, and those agendas will often clash with other monsters. If the PCs are laying low, or sneaking around, they will see monster conflicts. Smart PCs will then see ways to exploit such conflicts to advance their own agenda. Such conflicts also promote dramatic tension, and role playing.
The third is the element of "overwhelming force". The adventure should have the promise of a severe beatdown if the PCs play it too loose. Not just a boss monster that poses a great challenge as a final battle before the monte haul... I mean a persistent antagonist that truly cannot be defeated by the PCs, and the PCs know that, and they have to complete their adventure in spite of it. Facing some sort of overwhelming force like that really promotes creative play, as well as dramatic tension in the course of play.
The above three elements are key components of creating a good story line. Creating an engaging story line is, I think, one of the hardest, or rather, most subtle, parts about being a good DM.