Cimorene

sometimes you get fiction that doesn't bother to build up the emotional connection between the leads at all

I think of these as pairing buy-in stories. Because you have to be heavily invested in the pairing already for the story to work, but once you are, it really does. (Oddly, people tend to rec these as pimp-stories; i.e., the stories that will drag people into the fandom. I've come to the conclusion that it's a lot harder to determine what is and isn't a pairing buy-in story if you are invested in the pairing.)

This - not pairing buy-ins, but just general levels of shared knowledge and emotional connection with the character - is one of those things that contributes heavily to the fascination I have with AUs. Because there, you, yes, are dealing with a shared universe, but you've made changes to it. Even if it's a fusion, you can't really count on your audience knowing both fandoms. So I find it fascinating to track what people do and don't explain - like, cofax7 typically avoids explaining anything, AU, fusion, whatever. You'll pick it up as you go along, or you won't, but the story will work anyway. Whereas with, for example, the Reel SGA challenge, it went all over the board, and we saw a lot of examples of how varying degrees of backstory and assumptions about audience knowledge could work (and examples of how they didn't, too).

And fan fiction by writers whose original writing works for me tends to be different than typical FF (not all of it, but some of it); I have a FF litmus test that determines whether or not I'll read a fan writer's original work (actually, I'll generally read it anyway - it's more of a determining my level of anticipation or trepidation thing). It's proven to be fairly accurate, and it works best if you do it on the writer's FF AUs.

...Um. It just occurred to me that I'm hijacking this post, since this isn't what you were talking about, necessarily. But, yeah: I find the shared-world nature of fan fiction, and the effect that has on the writing style and content, to be fascinating. Obviously. It's just, I shouldn't try talking about it while I'm this sick. *makes mental note*