Hardcore violence, gore(not just simple injuries and blood), violent or sexual abuse of anyone really, graphic depictions of death(I'm more of a discretion shot, seen from behind/partially obscured, or showing the body afterwards) are the BIG nos because I get genuinely uncomfortable reading that, let alone considering creating it myself. Might be needed someday, but those are the lines that'll take a lot of money to cross AND be in a respectable context.
As for fetishes, aside from the illegal and offensive stuff that most people wouldn't do anyway, probably unusual bodily fluids, certain things that aren't gore but are equally as disturbing that I won't name here.