I've occasionally read or written sibling incest, and very rarely parent/child. As a general rule, the more fucked-up and dysfunctional a familial dynamic is, the more inclined I am to read incest fic for it. So, for a couple of Avatar: The Last Airbender examples, I'll happily read Zuko/Azula, but I find the idea of Sokka/Katara upsetting. The incest pairings I've written include Sam/Dean from Supernatural (extremely codependent and dysfunctional) and Miki/Kozue from Revolutionary Girl Utena (their relationship is distant, uncomfortable and has canonical incestuous implications).

One of the incestuous pairings I've read recently was actually your Jack/Claire Lost example! I was posting a Jack & Claire fic, so I was checking whether a Jack & Claire tag existed; in the process, I spotted the Jack/Claire tag and wandered in out of curiosity. It's a pairing with interesting potential to me because the characters don't know they're related, and they have absolutely no reason to think that they might be related - they're two strangers from different countries who happened to be on the same plane - so I can picture a scenario where they get together and then the revelation absolutely blindsides them. Jack has terrible self-control and strong potential for guilt, both of which are characteristics I enjoy a lot, and I think it's a pairing with good potential to bring out those characteristics.

I definitely consider Boone/Shannon to be incestuous, but that doesn't prevent me from enjoying the pairing; it's so interestingly fucked up!

If I came across incest in a fic without warning, it'd probably put me off, although I almost got used to it in Supernatural fandom back in the day. There are also plenty of non-incestuous pairings that would put me off if I wasn't expecting them, though!