One one hand yeah it's perfectly obvious why incest stories give people the heebie-jeebies, it's one of society's very dark taboos. For some reasons legitimate (potential exploitation of children, for one), and others more or less imaginary. But on the other hand it's not any different to me than any other fantasy to me. I've read my share of incest, it doesn't mean that for even a second that I'd sleep with my mom, or something. Granted I have a couple female cousins I'd admit an attraction to, but it's not like I'd ever act on that. I've also read plenty of BDSM stories about blackmail, kidnapping, slave auctions in the back of dark warehouses, scenes based around the concept of "non-consent but then she enjoys it" etc... hell, that stuff is more or less good fun, when done well. I can see to a point why some people have a problem with drawing the line between reality and fantasy with a subject like snuff or incest, but for me just because I enjoy it on paper desn't mean it needs to be something I enjoy in life. But it does seem that most people have a very subjective line in their mind for what is "okay" in fantasy and what is not.
I did go through an "incest story phase" about three years ago before I was into BDSM, a lot of "Mommy" stories, when what I was *really* looking for in hindsight was FemDom. It's not like I advocate incest in real life (though quite frankly, so long as it involves consenting adults I think it's nobody's business), but I admit I got a lot of kick out of quite a few stories on the subject.
Really, about the only "I really won't read that" limit I can think of for myself is scat play, which is also a very hard an unflexing limit for me in real life. I simply find it very repulsive... I suppose no different than some people are outright repulsed at an incest story, which I am not. I also don't like golden showers, though I'm not repulsed as much as by scat, it certainly is no turn on for me. I also try and avoid snuff, as I find it more "distasteful" as subject matter than "repulsive". I've never seen a snuff scene in a story that really turned me on, and I haven't read very many that list it as part of it's subjject matter. Oh, stories with very young children are an automatic no-zone as well, but this place doesn't have that. Teenage stories... it depends. After all, I was a horny teenager once too.
So I guess we all have our limits. But freedom of speech protects all of it, as I suppose it should. If you cut off one, what's to stop from another being cut off next? One of the things I like about this place is that most of the community seems very non-judgemental when it comes to what fantasies turn others on, and that's something to be commended and fostered.