I think the point about Shadowrun is well taken. I enjoyed the "fluff" stuff in those books as much as the crunch. It really added a lot. And I think that that's the challenge with the fluff - it has to add just enough to spark an idea, or solidify the rules, but not to belabor the point.
Planescape got this right on, so did Ravenloft.
I think Al-Qadim tried, but it had too much fluff in some of the boxed sets, and not enough crunch.
I think there are really two kinds of fluff. The first kind is what Shadowrun and Planescape had - bits of information here and there that added to the rules, made them seem real; made the environment seem real.
The second type is what should be avoid, and that is belabored rules about the government, political, economic, et al., about the setting. Al-Qadim really had too much of this second type in some of the boxed sets, specifically the City of Delights boxed set. I can remember reading through page after page of rules about the local court systems, and the judicial process in that book, that added absolutely nothing to my game, and indeed made a fun game even more complex, because now I had to keep track of customs and laws if I wanted it to be true to the system. Al-Qadim was perfect with the first book, and then slower started sucking more and more as each book came out that bogged down the setting further and further. They tried to get too detailed into the setting, and WoTC "boxed setted" it to death. Planescape didn't have this problem because the world was so expansive that they writers could only write fluff in broad, sweeping strokes, so it never got into all that boring detail. And that ended up being Planescape's saving grace.
I'm rambling, but I think the bottom line is that fluff should be just enough to provide a good picture of a setting to someone, but not so much that it causes the reader to alter their perception, or force him or her to make changes so that it integrates into their own personal setting easily.
For example, a lot of fluff in Bluffside is campaign generic, and doesn't require much work on the GM's part to include in their own game. Some of it, like the specific races, or the specific gods require a bit more work. Too much of the latter, and most people probably would have rejected it outright at the shear amount of work it would have taken to integrate the product.