For me, an instant turn-off is a character's beliefs, values, or moral/ethical worldview.
I'm actually pretty open-minded when it comes to an LI's personality, temperament, or the way they behave. I can enjoy all kinds of personalities, including difficult, morally gray, eccentric, or even problematic characters, as long as I can understand where they're coming from and find something interesting in their way of thinking.
But if an LI has certain beliefs or values that feel fundamentally distorted to me, or are just too incompatible with my own worldview, I immediately start thinking, "Yeah… there's no way this would work between us." At that point, I can still appreciate the character as a fictional character, but the romantic appeal just disappears for me.
Another big one is when an LI's reasoning or decision-making feels bizarrely stupid. Not necessarily because they make mistakes, but because the writing makes them consistently incapable of thinking things through, reflecting on their actions, or seeing things from a perspective beyond their own. I think I just really need to feel that an LI has a mind I can engage with.
I don't need him to agree with me on everything. In fact, I actually enjoy characters who challenge my perspective. But there has to be some depth behind their beliefs, and I need to be able to understand the logic that led them there. If I can't respect the way they think, it becomes very difficult for me to find them attractive, no matter how good-looking they are.
So I guess my biggest requirement is: I need to be able to respect your mind before I can fall for you.