Crunchy
"Crunchy" as it applies to gaming products:
Think of a Nestle Crunch bar. It's got a lot of candy-bar "crunchiness"...it makes a noise when you bite into it. It also gives a satisfying break with the candy bar itself, and puts a perfectly sized piece into your mouth for your own use.
In a similar way, think of a gaming book as a candy bar...lots of good stuff, right? If it's "crunchy," you can take out pieces of it and digest them without having to eat the whole thing. If the gaming product is "soft," it usually means you have to digest it all to use part of it. You can't pull it apart, make a clean break. Some unwanted part is always gonig to come with it.
This is usually "modular." It mostly applies to rules, as rules are fairly universal, but it also applies to other things. Rules can be quite "gooey," and flavor text can be quite "crunchy."
Take, say, a rule about how, in order to cast a spell, a certain type of spellcaster must have a holy item (say, a wand) dedicated to a goddess of magic. This is a very "gooey" rule. To use this rule, you have to have a godess of magic, and spellcasters nessecarily have to venerate her/him, or at least pay homage to him/her. It brings with it conceptions about the nature of magic and the world it exists in.
Take the same idea, and just say "these types of spellcasters must cast spells through a specially designed wand." This is very crunchy. You can "bite" it out of the book it's in, and use it elsewhere. Not nessecarily anywhere, but elsewhere, without including everything about the goddess of magic or the religiousness of spellcasters.
To give, perhaps, a better example, take two ways of using magic: the Sorcerer and the Wizard.
The Wizard is very gooey. To use a wizard, you make certain guesses about the nature of magic, that it's a study, that you have to research, that it takes nothing special to master, that spellbooks are essential, that intellectual types make the best spellcasters.
The Sorcerer is very crunchy. It's just "innate magic." It could come from dragons, Fey creatures, pacts with extraplanar creatures, nearly anything. It's very portable, and can make a clean break with much of the rest of magic, whereas the Wizard cannot (Most basic D&D classes are fairly crunchy).
The wizard is a caramel bar...bite it out, and more comes with it. The Sorcerer is a Crunch bar...bite it out, and it breaks clean.
Crunchiness, in general, is good because it allows replay on vastly different scales. I know of many campaigns that have no use for a Wizard, who can still find a use for the Sorcerer. Someone could use a wand-based wizard in a lot of places (like, I dunno, the Harry Potter world). But if that wand-based wizard has to be dedicated to a goddess of magic, it's limited to the archetypes it can fill.
Crunchiness is good because it allows homebrewing extensively. It's modular. Of course, it can result in a schitzophrenic document, or a lack of interesting text that causes it to read like a textbook, if done exclusively. A Crunch bar without the chocolate is so much rice krispies.
"Meaty" is a related term, but it's more often used when the gaming product is well-thought out, thick, substantial, without a lot of "fluff." Meatiness is good because it provides a lot of bang for the buck. The DMG is a product that isn't very crunchy (most of it's rules are very integrated with each other), but is very meaty. The Monster Manual, on the other hand, is quite crunchy, but not meaty at all (lack of in-game flavor for the monsters, etc.). Excessive crunchiness can sacrifice meatiness, and vice-versa.
Mimicing Greathouse's ideas:
Player's Handbook: Meaty, Crunchy
FRCS: Meaty, Gooey*
Hero Builder's Guidebook: Meaty
Splatbooks: Crunchy
Manual of the Planes: Crunchy
Oriental Adventures: Gooey, Meaty*
(*: These books make a good effort to be crunchy, but generally fall short. The Regional Feats, for instance, could be crunchy...but their tied with FR regions, making them gooey.)
Crunchiness applies to the modular-ness of the book, which mostly includes rules, but doesn't have to.