I was thinking about this the other day, but I can't remember what I was reading at the time to make me think it. Anyway, what can you just not bring yourself to write about? When I write, I have to write as the character whose POV it is. I have to be really in their head. So... I couldn't ever write incest - I couldn't imagine writing as a character who did that sort of thing, so I wouldn't be able to write it with any realism. Or animal cruelty. I just couldn't do it. I can write torture of humans though. And murder. And other horrible things. Weird isn't? What's your can't write/won't writes?