FLUNCHBrendan describes Affordances here. It's highly interesting and I think he's right. As +Richard G notes, Affordances (gameable elements, elements in a text that can be used as/translate easily to elements of play at the table) are basically that old "crunch"/"fluff" thing.
What comes immediately to mind is that the people I like, respect and look up to as designers and people-who-think-about-rpgs-on-the-web often work to maximize the number of Affordances in a given chunk of text while reducing or eliminating all together the non-Affordances.
For example, I'd suggest all of the below are meant to be pure Affordance:
A. +Alex Schroeder's +One Page Dungeon Contest
B. +Jack Shear's use of pastiche in setting description (see Gothic Earth or any of his other settings [there's a free pdf of Gothic Earth on his blog, check the right-hand column])
C. Noisms's 25 word description of Yoon Suin
D. +Wayne Rossi's exegesis of Volume 3 of OD&D
E. +Jeff Rients's Encounter Critical for Star Wars (or nearly anything Jeff does)
F. Both +Brendan S and +Talysman the Ur-Beatle seem to actively endeavor to limit their posts to only Affordances or ideas about/for Affordances (their blogs are here and here, respectively).
G. +Zak Smith's whole "let's find good things in old modules" thing (AKA reviews) is basically looking for really good Affordances in the otherwise barren field of over-written, professionally produced D&D modules.
D. is really an extended effort to demonstrate how much evocative stuff you can get out of a few mechanics and some tables without ever going into the usual setting book's wall of text (in other words: an argument for the power and elegance of an Affordance-heavy approach to design).
JOESKY TAXOK, I'm intentionally misapplying the tax here, but whatever.
SETTING GENERATION RULES
1. Write down the main concept of the setting. Can't be more than five words.
2. List everything about the setting. Or maybe do like a web-looking brainstorm thing. Whatever. You need at least 23 ideas and each idea should be no more than a couple of words, but the ideas should be evocative and can be names of spells, general things, a monster, etc. Just write them all down.
3. Take the 23 best ideas, the ideas which are most evocative, interesting and which most closely cleave to your notion of the setting and write them down as a list, one thing on each line.
4. Number them 2-24.
5. Find a blank hexmap. Somewhere in there is the home hex. This is where play starts. Roll 2d12 and write down the number on that hex. That's whatever idea is operative here.
6. Each time the players hit a new hex, roll 2-24 again and assign accordingly.
7. Make several sublists from your 23 options for rolling wandering monsters, city encounters, wilderness encounters, what kind of dungeon it is, etc, etc. Obviously, most of these entries are not necessarily going to point to something obvious (you've rolled "Magic" on a city encounter, what does that actually mean? [Figure it out GM]).
It'll be tricky at first but as more and more gets filled in/figured out, it'll become easier.