I'm glad you asked!
Here are mine:
1. The person thinks jokes about sexual assault (or other forms of violence) are funny or tries to minimize the seriousness of events in life or on the news that upset you. An example of them trying to invalidate your feelings on issues like this would be something like "get over it", "it's not that bad", "you're being too sensitive", or "the victim/plaintiff is obviously lying." An example of something supportive, non-invalidating they should say instead would be more like "everything's going to be okay", or "is there anything I can do to help you feel better?" Also, if they offer you tissues, a blanket, or your favorite food or hot beverage when you get upset, it's safe to say that they're a good fit for you.
2. They do drugs. (I imagine there's some decent gray area with weed, since some people use it for medical purposes or take it legally in moderation). I'm sure there are awesome people out there who are currently struggling with addiction. They deserve to get the help they need. But ultimately, getting help and recovering should be their number one priority. Dating can come later, after they get clean.
3. They've been known for making past partners or others cry. Not okay. Ever.
4. They hit you or someone else in front of you. Even if it's just one time, get out of that relationship as soon as possible.
5. You can't talk one-on-one without it turning into an argument. While I know some people love to have mini-debates, sometimes you just need to let your guard down and have an actual conversation with someone.
6. This one's more for me, but I think it's pretty helpful. I don't want to date any person who doesn't identify as a feminist. That said, there are people who uphold feminist values but don't call themselves feminists because they feel that the movement gives an unfair amount of credit to able-bodied white women while ignoring or excluding people of color, lgbt+ members, or transgender/non-binary people, etc. from the conversations. But if they shy away from the term for any other reason, it's usually not a good sign.
7. They use derogatory terms. For example, they call women the c-word.
8. Or they treat people differently based on their race, gender, sexuality, job occupation, appearance. For the job occupation one, a good rule to refer to is "if your date is nice to you but mean to your waiter, then they're not worth your time." (or something like that).
9. They make mean comments about your weight, your body, or how you dress.
10. They belittle your interests.
11. They're mean to your friends and family (or flirt with them. yuck)
I'm sure I'll think of more in the near future.