This is mostly about physical features.

If you have something that is special or unique about your appearance, something that sets you apart or gets you complimented, consider it a bright red flag if a guy you're dating isn't into it (in a healthy way, not in a fetishy way).

I'll use myself as an example. I have very long thick curly hair in a country of mostly straight hair. People remember me by my hair, it's my most often complimented feature, and practically a default compliment from men. It's also a little bit of a hobby for me, I enjoy caring for it.

Which is why it was odd to me that my ex never seemed particularly phased by it. Indifferent. I might as well not have hair. He didn't care if I cut it shorter. He didn't care if I grew it longer. He didn't care if I dyed it or not. He refused to take pictures of it for me, even though he knew it meant a lot to me.
Curly haired ladies know by now that the only time he ever said anything positive about my hair was...

.... when I wore it straighter. And of course he'd get defensive when I told him he never compliments it curly. I spent hours trying to figure out a way to straighten it damage free, just so that I'm attractive to him. Joke's on me though, he didn't give a damn - about me OR my hair.

Moral of the story: If a guy isn't wild about whatever you have going on appearance-wise, he likely isn't wild about the rest of you either.