I'm genuinely intrigued and would like to know what the attraction of Alternate Universe/Reality fandom is for those people who take part in it. I'm not talking about the stories that break away from canon, or that ignore specific parts of canon. I'm talking about the kind of thing where canon is completely thrown out and the fic writer starts from a completely new premise - whether it be that Xander and Angel are rodeo stars and there's no such thing as vampires; or that Dean's an FBI agent from a stable family and is not related to John, the prostitute (yes, these are specific examples used to illustrate a point - I mean no disrespect to the authors who are talented writers both).
Sticking to a visual medium here, could these kinds of tales be described as original stories in which the author has 'cast' the specific actors and used the character names we are, as fandom consumers, familiar with? Or is there more to it? Does the reader need the awareness of the canon versions of the characters to fully appreciate the story?
I don't tend to read a lot of these kinds of stories, which I'm sure is completely my loss for the most part, but I do know there are a lot of talented writers who write them. I am not in any way denigrating the writers, their stories or the concept. I am genuinely interested to hear the reasons why people like or dislike this kind of thing and what the reasoning behind it is.
So, opinions? I mention the Buffyverse and Supernatural because it's what most of the people who would be reading this are most familiar with, but I'm guessing that all fandoms have their variation on the AU/AR theme so please, don't just restrict yourself to those I mentioned.