I don't really have any anime that I drop. I wouldn't call myself picky, I watch a pretty wide variety of anime, SoL, romance, sports, fantasy, sci-fi, etc. But I have a set of turn offs that make me avoid some anime completely. Usually, the posters themselves are dead giveaways of what I wont like. If I've yet to really make my mind about one, I'll normally check its MAL rating. Anything below 6 I'll avoid unless a friend or someone I follow recommends it.
If I did watch something I disliked, though, I would rate it a 4. Just checked my MAL and my lowest rating currently is a 5 for Final Fantasy Unlimited.