...is the stuff which either fills in space or just becomes the very space which makes something either lightweight or pretty filled in the right places.
Just cruised around the forum boards of RPG.NET for any interesting reading for ideas and reviews and came across this thread. The mention of crunch (game mechanics, rules and math for the not-so-gamer savvy) made me recall an entry I did on my blog here. Its a bit of debate necromancy as to the GNS model of gaming since time immemorial when critical analysis of games have brought to bear on how games were conducting in spite its very obvious and earlier tabletop wargaming roots. But anyways I will not dwell on something that's been-there, done-that and the ever usual spark of violent combustibility for a flamewar of sorts, I'll just examine and discuss what fluff would do or help in the arsenal of items a GM would use besides the toolbox of his ruleset of choice.
Fluff which is usually defined in the gamer community usually connotes to the background and setting info which would cover a lot of info regarding the game world from macro to micro aspects of existence and its many inhabitants. This is usually taken as is by the GM and players to use and base their decisions on the PCs and how NPCs and campaigns are forged. In other cases, the fluff would be very dry or light in material which leaves a lot of interpretation or guesswork to do for the GM to wrack his brains over to no end and also leaves a potentially big loophole for some enterprising PCs to exploit and manipulate to no end besides their apparent abuse of the system they know inside and out, especially if the GM is not fast on the uptake with this. A better way to approach it is that fluff and crunch have to go hand in hand in some measure to better illustrate how it works in the game setting and how the game rules or stat illustrate it without being overpowered by either end of fluff or crunch.
Some sourcebooks for what appears to be for exclusive game systems it operates under can basically be retreaded into other game systems of a more suitable preference, if systems would allow for it in-genre of course. Example of such books to idea mine, port and transfer would be like Century Station Sourcebook for Heroes Unlimited by Palladium Books, Unnecessary Evil for Savage Worlds by Pinnacle Entertainment Group,
Transhuman Space for GURPS and standalone (with built-in GURPS Lite) by Steve Jackson Games, Centauri Knights for BESM by no-longer-active publisher Guardians of Order. Sometimes a genre-specific tie-in would help ease the transitioning to some manner as long as conversion issues are not to problematic like example of transplanting a superhero setting or ideas from its native system to a more familiar system of the GM's choosing, complexity taken into account as well.
So basically fluff can be both an attraction and a deterrent as well the way the system or crunch would be in the presentation of a given game. If done in the right proportion or properly moderated or presented, it wouldn't prove to complicated or daunting for the reader, whether it be prospective GM or player.