A set of modules that can be run as a series or seperately (perhaps having sidebars or notes on how to use the modules as part of a series &/or individually).
(Still going with the module series concept...) A set of modules that provides a good variety of adventuring genres/milieus (a dungeon crawl, a wilderness survival epic, exploration, sea travel, a city adventure, a mystery, a horror adventure, a comedy, etc.).
Recurring villains (Maybe, if feasible, a group of recurring villains that can be pick-n-pulled from a set of pre-genned NPCs to specifically counter the PCs--a Linear Guild to the PCs' Order of the Stick). Ideally, they should be easily genericized NPCs so the modules can be used in most campaign settings--like clerics with domain choices that can mesh in easily with existing domain selections for deities of campaign settings like Greyhawk, FR, & Eberron, so the Evil NPC cleric could feasibly be a cleric of Hextor, Bane, or the Blood of Vol, respectively.
How about modules with "optional" sections, like side quests in a CRPG--they don't have to be done to complete the module (or module series), but they provide a nice perk if they are. Perhaps even provide notes on modifying the optional sections in case the PCs try to tackle the side quest early on or later on (i.e., not at the exact PC levels it was initially written for).
One thing I've noticed is that some adventures/modules generally tend to assume that the "classic 4" classes (fighter/rogue/cleric/wizard) will be covered by the PCs. I'd like to see modules that aren't written with that assumption in mind, & are still challenging/rewarding (basically, by not having the solution to an encounter/problem require the PCs to have a specific Feat, X ranks in a specific skill, or access to a specific spell, racial ability, class ability, or stat value to overcome).
