Zak posted about D&D as a modular platform, which is alway how I've viewed it.
Half the time you see a new table or chart on this blog it's because I went "Ok, for my next session I'll need a fortress--hey why don't I spend the amount of time I'd spend designing that fortress adventure making a little subsystem to generate fortresses." This isn't always the best approach but when I do it that way it generally works out--the brainstorming I do to get the entire table might give me ideas I wouldn't have thought of if I'd just been focused on building one location. And, naturally, it saves a lot of time down the road.
Computer programming nerds call this "writing reusable code."
The quirky little subsystems are one of the things I love best about OD&D. Unfortunately, few of those subsystems survived to later editions and the retro-clones.
I'd love to see an anthology called Quirky Little Subsystems. Start with Swords & Wizardry Whitebox or Labyrinth Lord: Original Edition Characters as the platform. The book, then, would contain dozens of modular subsystems by various contributors. Individual referees could stick them together or adapt them as desired.
This isn't quite a call for contributions, but I'd enjoy editing and typesetting Quirky Little Subsystems. Is there any interest in such a project? Please comment.