Being on friendly terms with past partners is another one. Or for that matter cherishing the memories of the good times with those partners. A lot of people seem to think that after a breakup you should hate your ex forever and burn all pictures, throw away any object they ever gave you,... but that is actually quite unhealthy (unless abuse or stalking or similar things were involved of course). If a relationship does not work out that doesn't mean that other options, such as friendship, might not be on the table and even if they aren't that doesn't mean you can't treat each other like regular acquaintances when you randomly meet somewhere. Obviously they might not be an option immediately after a breakup but once time has dulled the pain a bit a friendship is absolutely possible with someone you initially shared enough interests with to try a relationship.

When they mention that someone else is attractive. This is often seen as a red flag by people with insecurities but really it means your partner trusts you enough to actually mention such attractions. The state some insecure people want is that their partner is never attracted to anyone else but that is completely unrealistic. So the actual choice is between honesty and lies. And you do not want your partner to have to constantly watch every word they utter around you to coddle your insecurities as that will likely lead to worse communication between you in general. This goes for other topics as well of course but jealousy inducing ones are very common.

To be nice or friendly with kids.

Not talking all the time when spending time together. Being able to just quietly enjoy each other's company sometimes is actually a good thing since it allows both partners to relax without constantly worrying about keeping their partner's attention or keeping them entertained.

Taking some time to calm down during a fight if getting angry/sad/whatever.
The other party might think that you are running away.

Make sure they know that you continue once calmed down.

Being underemployed. As long as they meet their obligations, I applaud people who don't live for work.

If a person can readily describe their failings it could seem like a red flag because they have failings, but everyone has failings and being aware of them is a positive.

Not having a Facebook profile. I've had someone initially refuse to associate with me on the basis that they couldn't investigate my life beforehand.

I just laughed and asked them how they managed to survive before the Internet (we were both old enough). We both got over the weirdness of the situation, built a robot, and were friends for a while before they moved away.

Making life choices different from the societal standard (e.g. not wanting children or not wanting a marriage). Sure, if your own desires are incompatible with that you might need to find someone else but a lot of people who do go with the societal standard actually just do so because they never thought about alternatives and have a rather romanticized notion of that default option and might still grow to regret it later which can then often lead to breakups/divorce if that only happens to one partner in the relationship. People who make different choices at least thought about what they want. Basically you want a partner who has already thought about these and not one who only discovers their actual preferences on these options a few years into your relationship.

Any size.

If you're colorblind.

For people who value reading: if they have no books on their shelves. They might be avid readers of ebooks, or just use the library.

But this should clear itself up with a rather simple discussion started by mentioning a book you read recently.

All these stupid "ignore them to seem attractive because interested = unsexy"

Not being a virgin anymore? Thats something good too.

Having actively broken up a past relationship, knowing barriers.

Hanging out with friends rather than you sometimes, which is really important "relationship time management"

Being a healthy weight - as misinterpreted by shallow young people that "want dat thigh gap".

Them being happy for how they are, like myself being happy while larger.