For starters I am happily single and I just want to say to all on thread if you didn't already know that being single is a perfectly valid choice and certainly better than being in a bad relationship! I really think we need to be supporting women and girls to know that being single can be a great choice
That said
I have ocd so hygiene is a big one for me can't stand scruffy. Grubby people
Having no real friends
Omg my exh! He'd be pally with colleagues (army) but when we moved he didn't stay in touch with people and they didn't with him it was v superficial
having no real passions/ hobbies
Also exh except for rugby. He only joined army as his dad was army and he basically cba to even think to try anything else
Inverse snobbery/snobbery/inferiority complex -
Ex had left school at 16 having scraped through GCSEs just to get enough to join army, hated school but not stupid he had a brain he just didn't like using it!
I'd also left at 16 but went to eve classes to do A levels and when we met I had just applied for uni.
But his family were also more middle class (his dad an officer mine not) compared to my own very working class family and were better off
So he had issues with me being "brainier" than him which especially came up when we argued as he complained about me being able to think quicker than him and use "stupid big words there's no need for"
But he also looked down on me and my family for being working class/poor:
My hard no's are
Heavy drinking
Drug taking AT ALL
Addictive behaviour aside from substances too
Money - I hate stinginess but I also think being a spendthrift is also something I would avoid. Generosity of spirit is important, but also being savvy money wise is very appealing. Basically I wanna date Martin Lewis! 
One person I dated (briefly!) paid every restaurant bill to the penny, left a tip but EXACTLY 10 % every time and only if the service was amazing. They hated using their card and did almost everything by cash.
Also agree with all those saying not treating people serving you well. Anyone who treats people who are "lower" than they consider themselves to be is not worth your time. One date with a guy who actually clicked his fingers at the waitress - nope! I refused his offer of a lift and apologised to the waitress appalling behaviour! Bit of a tricky one as he was a friend of a friend and I was asked by friend why I wasn't up for a 2nd date but I was honest with friend who was also appalled (they knew each other through a hobby hadn't been for a meal together)
One I haven't seen mentioned but I think is increasingly an issue in our insta society - people who are more concerned with appearances than the substance of their lives/relationships