I can pretty much tolerate any and all kinds of things a story throws in my face, but if I like it or not depends on how the theme is used. If I see rape or wincest done just to turn people on, for example, I just consider it pretty normal, especially in a VN, but if it's done right rape can lead to glorious mental breakdowns, and wincest to heart-rending moments of "this-love-is-impossible-but-we-do-not-care", much to my delight. Bonus points if whatever theme it is gives me a good ol' shock.
The exception to the above is gore and cannibalism. I enjoy them even solely for the lulz, and if they are done with care, love, and a pinch "let's screw the reader" it can lead to truly unforgettable moments such as (be careful, spoilers ahead for Muv-Luv and Saya no Uta!) or the