The Golden Rule
The only one thing in human relationships of any kind I consider being ultimately bad. It's violating a direct 1:1 agreement. If the person does it once and acts like it's not a big deal, it's a red flag. If the person does it consistently, it's a hard pass.
Different people have different standards, tastes and expectations, are repulsed by different things, and at the same time, can be totally fine with something others find repulsive. But the one thing among them is universal.
You can't build anything with someone you can't rely on.
Sounds like something painfully obvious, yet overwhelming majority, surprisingly, fail to comply with their own words. If you agreed to meet up with someone at precise time in precise place, showing up 5 minutes later is violating of promise, and is contemptful in regards to feelings of the other side.
Something barely anyone thinks of.
The polar example is lying: not only I don't think it is ultimately bad, I personally would even prefer to be free to tell small lies here and there, for the sake of maintaining personal safety, comfortable environment and seamless conversation. I do it all the time, and am fairly open about it.
But if you promised to always tell the truth (overall, or in regards to certain topic), and then you continue to mindlessly bullshit as if nothing happened, that would definitely be a horrible thing to do. Needless to say, that always goes both sides, genders or social roles of people involved do not at all matter.
People are different, and they are fine with different things. We need to learn to discuss as much as possible honestly and in person with people we are planning to do some business with.
Fewer silent assumtions, more direct and honest negotiations.
This is the way.