I've decided to elaborate on this since over the past few months the meaning of this word has twisted to include roleplayers and other labels that aren't fluffy, but poserish.
Fluffy Bunny, or Fluffbunny, is a pejorative expression used since at least 2001 in Wicca (and in Neopaganism generally) to refer to adherents of the religion who are thought to be superficial or faddish. They are considered to dislike darker elements and emphasise goodness, light, eclecticism and elements taken from the New Age movement, or follow it as a fad.
The primary definition of a Fluffy Bunny is one who refuses to learn, refuses to think, and refuses to consider the possibility that they could possibly ever be wrong. Generall, they find one book, author or website and follow it as if it were the holy word, frequently denouncing anything that disagrees with it as obviously false. Fluffy Bunnies rarely get past the defense of "Because [insert favorite author here] says so." Sometimes they don't even get that far, responding to any and all criticism with something like, "You're just trying to persecute me!"
Related terms used with similar meaning are "Insta-witches", "McWiccans", "One-Book Witches", "Wicclets",  "Playgans", "White light-ers", "Baby Pagans", and "Weekend Witches". In the 1960s, Robert Cochrane used the term "old lady brigade", to describe others overly fixated with sweetness and light.
As you can see, roleplayers and the other labels don't exactly fit in this term. I personally have more respect for a fluffy than for a roleplaying poser. I hope that clears up this term for everyone.
Hanna, Jon (2010). "Chapter Four: Training, Standards and the Anti-fluffy Backlash". What thou wilt: Traditional and Innovative Trends in Post-Gardnerian Witchcraft. Cathair na Mart: Evertype.
Hanna, Jon (2009). "IV Training, Standards and the Anti-fluffy Backlash". Traditional and Innovative Trends in Post-Gardnerian Witchcraft. Dublin: Hackcraft.Net.
Noble-Bayer, Catherine (2002). "Fluffy Bunnies". Wicca for the Rest of Us.
Pathfinder (2004). "Fluff vs. Hardcore". Stuff and Nonsense.
Howard, Mike (2001). "Chapter One". The Roebuck in the Thicket: An Anthology of the Robert Cochrane Witchcraft Tradition. Capall Bann.