This is not the meta you're looking for
I don't really go for the wincest, which is like SHOCKING, I know. Usually I'm like YAY INCEST! I love the twincest, I like Don/Charlie, etc.
1. A big reason I like incest normally is for the hidden, clandestine aspect. Hiding it from friends, family, and society. Always lying. The shame and angst from that. With wincest there's no one to lie to. They're completely on their own, not a part of any society.
2. I want the incest to be the main angst in the relationship. The defining aspect of the relationship. In "Parallel Lines" Don/Charlie worked so well for me because that was the central aspect of their relationship.
With Dean and Sam, they have SO MANY good emotional issues and angst and stuff, that any sex between them seems like a distraction. Like I've read good, hot Wincest sex, but it always seems kind of plopped down in the middle. Like I can't follow the reasoning to get to the sex. I want to explore the real issues between them in canon, and adding sex or frustrated sexual angst to it just doesn't do much for me when they already have so much angst between them as it is. That's not the central angst of their relationship.
With Don/Charlie, they didn't really have any big angst in canon, so it was easy to graft the incest on to their relationship to make it more interesting. (Or with Fred/George, for example, it gives depth and angst to their often canonically flippant personalities.)
Because if the incest isn't central to their relationship in the fic, then we aren't reading a slash fic. A romance fic. And many wincest fics *do* make the incest central. And that's why I can't believe them. Because I don't feel that it *should* be central in spn. There's so many other canon issues and emo porn waiting to be explored.
That's what I want from my spn fics. But maybe it's just that I have yet to read a wincest fic that really explores all those issues, and moreover, uses the incest to explore those issues, has sex with a narrative and emotional purpose that rings true to canon to me. But then I haven't read a whole lot of wincest fics altogther. I'm waiting to explore the giant reclists on a rainy day. A very very long rainy day.
So that's why I'm mostly ambivalent about wincest so far. I can take it or leave it. Partly because I don't feel that every emotional closeness issue *has* to be solved through sex, but mostly because it hasn't yet given me what I want from spn fics. (Like I loved the gen "Parts of Speech.")