Lack of steady income. A lot of people feel that that's shallow, but let me tell you, having to front for someone a lot and always worry if he's going to be able to contribute at all (rent, bills, going out to dinner, hell even just having christmas together) gets really old really fast.
Talking to an ex doesn't bother me: I talk to my exs all the time and they're some of my best friends. Relationships change over time--this notion that because you're no longer romantically involved means you have to cut them out of your life is dumb lol . If it's a case where it's toxic for you to be around them, then by all means do so! But if you're in a spot where you're cool with just being friends (especially after having already spent so much time together), then go for it.
Drinks too much alcohol is also kinda a turn off for me. I've had someone puke in my bed before, and I often end up playing the role of responsible adult who has to navigate everyone back to safety, which, as I said before, gets old really fast
I suppose infidelity is a fairly strong offense, although I've been cheated on before as well and it didn't end that relationship since we discussed it.
No common interests would be really hard though--like pinkmakona said, what am I supposed to talk to you about? We don't have to like EVERYTHING, but some core interests is almost essential. However, liking too many of the same things makes me feel like I'm losing autonomy in my identity--it's healthy to do things separately and like other things that your partner does not enjoy.
Drug use really depends. I personally want no part in it, but good luck finding gays that don't do pot lololol. There are plenty of people who do it and that's all they talk about as a guise for having 0 personality, but occasional use in their own home is fine.
Emotionally guarded is kinda up there for me, although I am able to make really guarded people open up fairly easily so you'd need to try really hard to completely shut me out lmao. I'm emotionally guarded myself, so if I'm as annoying as I imagine I am, I wouldn't want to put up with it either.
One thing that I think is missing here that might apply is being of a different age group. There are plenty of 18 year olds who don't date anyone under 45, and there are just as many 45 year olds who refuse to date anyone younger than them. I find that most people like to stick within +/- 3 years, depending on how old they currently are, but I can understand that mentality.
Using myself as an example, someone 3 years younger than me is probably still in college and hasn't really had a chance to be a real adult. We come from different phases in our lives, have different goals, and have different priorities that don't necessarily line up. Someone who is 18 is only exponentially different, and someone who is in their late 30's more often than not in a different place as well. It's less of an "ew he's old!!!" thing than "I don't know what to talk to you about because you could like be my dad."