Being rude to waiting staff, which reveals narcissism, entitlement, snobbery and general arse-brainedness. Conveniently, it's a tendency which often reveals itself on a first date.

unkind to service people, children or the elderly

Lack of compassion, manners and unable to empathize. Big No No’s for me. Oh, just one? ?
Negative personality.

A mustache thicker than mine. I know it's shallow, but that is where l draw the line.

Pettiness

If they think they have no flaws or if they think that they are completely flawed.

A lack of ambition. Don't see myself being with a person who doesn't have eyes set on something to achieve

No books.

Mine would be when a woman asks me if I would be willing to go to therapy on the first date.

If she says she isn't an affectionate person. If that's the case, I might as well stop 'ya there 'cause this guy needs physical touch to be happy and feel loved.

They always show their pet in every photo and is always with them. To me that is telling me that their pet will ALWAYS be more important to them than I would ever be and if there were ever a conflict between me and the pet, I would have to go. Why get involved with anyone like that to begin with. Save yourself a lot of trouble and fine someone who will put you above all else.

I've not experienced that but my red flag is lies. I've had so many men lie to me, mostly just trying to make themselves look good. Later I find out that whatever they brag about doesn't exist.

A lesbian or bisexual who won't fully accept her own sexuality or believe that her relationship with.me is 'a real relationship.'

Smoking. I grew up around smokers and I don’t want to deal with that again.

Pushy manipulative types. Red flag. I have an ex who still tries to make me do things. Now his new thing is trying to make me please him by buying a road bike. And I should buy it NOW. Because he wants me to buy it NOW. And he wants me to spend a couple thousand on it. And he wants me to buy it at a certain store that he likes. He'll keep sending shitty manipulative little texts about my buying a bike as though pleasing him is something i still care about. I don't know what I ever saw in him.

If they don't like animals are controling or nagging...and only talk about themselves.

With my mental disorder if I didn't have therapy, and actually use it I would probably be dead.

As far as a red flag,.. I guess it would have to be fake boobs. To me that just screams of some one who is not comfortable being who they are. That kind of insecurity is kind of a big turn off for me.
Just saying.

Huge red flag if she plays the ukulele. ?

Serious answer: I'm wary whenever anyone is too interested too quickly, e.g., professing love after only a couple of dates or a few conversations, or starts talking about serious commitment before we've really gotten to know one another, or claims to be dying from cancer and blames me for it (actually happened, and we'd chatted only a handful of times over the internet, but that might have been an attempt at catfishing). So, I guess it comes down to anything that shows manipulative behavior of various sorts.

Lack of empathy for human dignity. I know we all have our differences, but there are some things that demean humanity. And if you cannot at least acknowledge and honor another's, I'm out.

A man who's used prostitutes.

I am a fantastic listener, but it's nice to be listened to on the few occasions I would like to say something to add to the conversation.

Signs of narcissism, it goes against the grain of everything I believe to be attractive about a person. The qualities I prize are humility and gratitude.

Signs of entitlement.

Lack of empathy. E.g., I do have some female friends who voted for you-know-who. Even if I did find them attractive (which I did in a few cases), I would never, ever, EVER date them! It's just too much to forgive.

My biggest red flag is when all they talk about is them. And no matter what you say, they one-up you. I call these people me-monsters. Or one-uppers.