There are many reasons people end up being late for dates. Sometimes they're simply trying to look their best and spend too much time at home in front of the mirror, and sometimes they get caught up in extraneous stuff that slows them down. In either case, being late creates a bad impression. When you're late, it puts your date in a bad mood from the start and that initial irritation could ruin everything.
Everybody wants to impress when they're on a date, but you should avoid overdoing it with dishonest claims. Sure, it may make you look good in the short run, but eventually you're probably going to have to tell the truth. Some people are particularly good at recognizing when people exaggerate, and they're likely to see right through your ruse.
When you're on a date, you obviously feel a need to tell your date a little about yourself, but don't monopolize the conversation with constant descriptions of your past and exploits. This makes your date feel that you have no interest in their lives. You need to make sure the conversation is more evenly balanced, and if you feel like you're the only one talking, that's probably a bad sign.
Talking about Previous Relationships
Most of the time, the last thing your date wants to hear is a lot of talk about your past relationships. It makes them feel like you're still in the middle of another relationship, and it shows that your focus is still on something in your past instead of your future, which gives a bad impression.
Moving Too Fast
It's wise to move things slowly, especially in the beginning. If you immediately start talking about being in love or start pressuring your date toward a physical relationship, it can make things uncomfortable. This is the kind of thing you have to play by ear because people have their own personal tolerances and expectations, but a conservative approach usually works better in most situations.