For me it would be:
-Anyone who doesn't like or is cruel to animals. I could never be with someone who treated animals poorly or thought they deserved to die for any reason, because I don't think it's fair and no animal on this earth deserves it. I would probably have trouble being with someone who worked for a company that directly destroyed animals or the environment as well, like a construction company, zoning commissioner, slaughterhouse worker or owner, etc., unless it wasn't possible for them to work any other job for some reason and they were trapped in that position but didn't like it.
-Anyone who talks about exes or looks at other girls too much. I would be able to deal with a guy thinking a celebrity was hot or something, because I grew up immersed in that sort of thing myself and I think that's reasonable, because you're usually not going to meet those people IRL and they're not normally a threat. However, if a guy is checking out other girls--even one--IRL, then that is a threat, and it would make me more uncomfortable because I would feel like "Why do you need to look at anyone else if you think I'm the end-all be-all of hot?" etc. Unless they told me specifically that they're honestly from a religion or culture where multiple wives is acceptable, I would just think they were a selfish asshole and not want to be with them anymore. That's why I think the divorce rate is so high--people hook up when it's not with "the one" person that is the end-all be-all for them, and they end up cheating because of that and it's a huge fucking nightmare
-Anyone who told me I'm a conspiracy theorist when I express my less-common beliefs about things. Though I believe that they're completely practical and make sense in the scheme of things, many of my beliefs about RL issues are unconventional, and rarely expressed in popular culture because the entire "system" that we have going on in society would collapse if literally everyone started believing those kinds of things. But, some people (and a few people I've known) have been raised all their lives being ingrained into society so much that they've actually told me that I'm too paranoid and "think everything is a conspiracy" in a condescending manner. I could never be with someone who thought I was crazy for thinking all those things, because that would mean if I ever had a chance to do something about something that was wrong in the world, and it involved such crazy thinking, I know they would never be willing to help me with it. I mean I guess some people could do the whole "dragged along" thing like you see in the movies (think "National Treasure," where the girl doesn't believe there's a map on the Declaration but she still ends up helping anyway), but it would be better if they just full-on agreed with me to save a vast amount of time and effort.
Also this. I've had a platonic guy friend or two do this to me, and I really didn't appreciate it--they came to me about a personal issue first, which I would expect establishes a precedent for communication about such things, but when I wanted the same kind of listening back, they weren't willing to do it
That sort of thing is bad enough when it's just a friend, so I would never be able to deal with it in a romantic relationship.
I've seen some people say that they wouldn't want someone who's too "clingy" either, but you do have to understand that there are some people out there (like me) who act like that sometimes because they have legitimate mental disorders, such as anxiety disorders or OCD, where it's common to worry about every little thing and you just can't help it. So if that's literally the only thing keeping you from spending the rest of your life with a person, but you're crazy about them otherwise, then I would definitely recommend trying to find out whether they have a legitimate clinical problem or not. Some people could certainly act like that just because they want to be a controlling pain in the ass, but some people really can't help it, and might be willing to get treatment for it if they care about you enough, and don't want to lose you over it either. And if they don't realize that they're annoying you, you should definitely talk it over with them first; don't just up and leave them out of the blue, or scream their head off and stop talking to them.
-Anyone who doesn't like or is cruel to animals. I could never be with someone who treated animals poorly or thought they deserved to die for any reason, because I don't think it's fair and no animal on this earth deserves it. I would probably have trouble being with someone who worked for a company that directly destroyed animals or the environment as well, like a construction company, zoning commissioner, slaughterhouse worker or owner, etc., unless it wasn't possible for them to work any other job for some reason and they were trapped in that position but didn't like it.
-Anyone who talks about exes or looks at other girls too much. I would be able to deal with a guy thinking a celebrity was hot or something, because I grew up immersed in that sort of thing myself and I think that's reasonable, because you're usually not going to meet those people IRL and they're not normally a threat. However, if a guy is checking out other girls--even one--IRL, then that is a threat, and it would make me more uncomfortable because I would feel like "Why do you need to look at anyone else if you think I'm the end-all be-all of hot?" etc. Unless they told me specifically that they're honestly from a religion or culture where multiple wives is acceptable, I would just think they were a selfish asshole and not want to be with them anymore. That's why I think the divorce rate is so high--people hook up when it's not with "the one" person that is the end-all be-all for them, and they end up cheating because of that and it's a huge fucking nightmare

-Anyone who told me I'm a conspiracy theorist when I express my less-common beliefs about things. Though I believe that they're completely practical and make sense in the scheme of things, many of my beliefs about RL issues are unconventional, and rarely expressed in popular culture because the entire "system" that we have going on in society would collapse if literally everyone started believing those kinds of things. But, some people (and a few people I've known) have been raised all their lives being ingrained into society so much that they've actually told me that I'm too paranoid and "think everything is a conspiracy" in a condescending manner. I could never be with someone who thought I was crazy for thinking all those things, because that would mean if I ever had a chance to do something about something that was wrong in the world, and it involved such crazy thinking, I know they would never be willing to help me with it. I mean I guess some people could do the whole "dragged along" thing like you see in the movies (think "National Treasure," where the girl doesn't believe there's a map on the Declaration but she still ends up helping anyway), but it would be better if they just full-on agreed with me to save a vast amount of time and effort.
No empathy: I like guys who will listen to what I have to say and care about me. I usually listen when people talk to me about something so I expect the same back. So if you don't care about anything, anyone or don't listen then that's a deal breaker.
Also this. I've had a platonic guy friend or two do this to me, and I really didn't appreciate it--they came to me about a personal issue first, which I would expect establishes a precedent for communication about such things, but when I wanted the same kind of listening back, they weren't willing to do it

I've seen some people say that they wouldn't want someone who's too "clingy" either, but you do have to understand that there are some people out there (like me) who act like that sometimes because they have legitimate mental disorders, such as anxiety disorders or OCD, where it's common to worry about every little thing and you just can't help it. So if that's literally the only thing keeping you from spending the rest of your life with a person, but you're crazy about them otherwise, then I would definitely recommend trying to find out whether they have a legitimate clinical problem or not. Some people could certainly act like that just because they want to be a controlling pain in the ass, but some people really can't help it, and might be willing to get treatment for it if they care about you enough, and don't want to lose you over it either. And if they don't realize that they're annoying you, you should definitely talk it over with them first; don't just up and leave them out of the blue, or scream their head off and stop talking to them.