My favorite aspects of fluff is setting material. What makes this world tick? Give me detail on the major points that is integral to the setting (such as technology for Sci Fi or gadgets for Steampunk games etc.) and how they define and shape everyday life. What are some of the more interesting places and characters in the world? And my favorite way that stuff is presented is in-character. Eclipse Phase, The Laundry, and Low Life do this well. All the fluff is presented by characters within the setting who know what they're talking about, and more often than not, memorable.
My least favorite is short stories. They're just a wast of space to be honest. I'm also not a big fan of massive info overload that stifles my creativity such as an overly detailed world (FR I'm looking at you). I'm going to throw adventures and pregens in here as well. I can do that all on my own thank you very much. Hell, if the setting material is interesting and provides me a very good picture on the world and what adventures to run, that stuff should come naturally anyhow. As for presentation, hmm... I haven't found any that gets on my nerves. I guess something that is too dry, or the in-character voice is not that appealing.