Nothing's really taboo for me, mentally. Writhing mass of weeping-baby-faced snakes in my brain.
But there are things I probably wouldn't write about because they just don't interest me. Professional sports (unless I make one up). Church services.
ETA: Come to think of it ... there are a lot of things I can't help thinking about, but probably wouldn't write much about, just because I find them depressing without being entertaining or leading anywhere. Facts about the world; the general hopelessness of existence, etc. They'll always be there in the background, but probably not explicated.
(Oh -- and that "torture porn" crap.)