For example, I had a really hard time getting into the Nantucket series.
You know, that's one of those fics I just haven't gotten around to reading yet. It's on my To-Read list, and I've listened to some of the pod fics that have been done for it, but they didn't have be running over to read the whole thing. That's the one where John is hurt and moves to Nantucket, and has a dog named Cash, right? I think you're right about the characters missing that spark, the bits I had listened to had me thinking what is this? because I couldn't really recognize the characters or see where the story was going. But that could totally be because I've only listened to parts. One day, I will actually try reading it. :P

I think everyone has a set of traits that define their favorite characters. When those traits are contradicted or missing, it ruins that particular fic for that particular person, regardless of the quality of the work
*nods*
I've had discussions that skirt this topic, but hardly anyone comes out and actually talks about it. Replying to comments on this thread has really made me come to terms with the kinds of fic I read; which of those traits I regard more highly than others, and already I can see where they clash with those that others hold as important.
Like that link I had given to you about that discussion I had with amireal where she was saying it throws her out of a story if it seems improbable (how she lives in the SGA fandom with that idea I'm not quite sure...), bad medicine, societies that look like they'd implode if actually lived, reasons behind actions and plot devices; those are things that heavily influence her like or dislike of a story. But to me, some of the more improbable situations appeal to me. I love romance, and courtship, I nearly died of glee when sgaflashfic held their harlequin challenge, and I have the harlequin comm on my flist. Mostly though, I want a fic to make me *feel*. That's what's most important to me. It can be the most outrageous piece of science thought up, but if it makes me laugh? Still good. It could have the character's be really OOC, but if it makes me cry? I'll love it. Because that's what I look for in fic. Something like Freedom's Just Another Word For Nothing Left to Lost didn't wow me the first time I read it, because it wasn't able to hit me emotionally (when I listened to the pod fic it was able to get me, but the first time it just left me feeling kind of meh) but something like Edges? It had me crying as I read it, and for most of the night after I finished. Every time I read it, it makes me cry, and I *love* it for that.