It's been a long time since I was dating, but for me the big ones were:
Complete negativity about everything. Moaning about their job, their boss, their family, their ex-partners, money, their friends, missed opportunities, broken dreams and various other perceived slights. Often went hand-in-hand with being rude to waiters etc as the world was apparently against them. Not everyone is where they want to be in life, especially in their 20s, but when it's endemic it's just tedious, especially when it's a stranger and the evening is supposed to be fun.
No effort to make conversation. I get that not everyone is a natural but still, bring a couple of ideas for a dull moment.
One single interest. That they bang on about all night. Obviously great if you have a mutual interest, but if not it's something that put me off a few times. If they couldn't tell their date was bored rigid after taking at them for hours while they try and get any other topic in edgeways, then I was out and glad for the silence on the way home.
One thing I did do towards the end of dating, before I met my wife, was to start meeting for coffee in the afternoon as a brief 'introduction' to see whether an evening 'date' would be fun for both of us. So many dates clearly weren't going to work out after the first 30 mins that I got fed up with commiting to dinner and an evening out. The first date (if you're strangers) you're really just sounding out the superficial first impression stuff anyway, and that's the stuff that was usually an immediate interested to know more/not interested in my mind.