when i think about unsafe divers, i think about the line from top gun: "your ego is writing checks your body can't cash."
i think attitude has the most to do with it. Someone who is thoughtful and prudent and is capable of a reasonably objective evaluation of their own capacity is unlikely to be an unsafe diver. Someone who is cocky, careless, arrogant, dangerously cheap, or badly disorganized is unlikely to be a safe diver. Someone who doesn't respect his buddies or respect their requested limits for the dive is not, to me, a safe diver. Someone whose behavior is unpredictable or who does not have themselves under control is an unsafe diver.
I've only done two dives ever with people i thought, by the end of the dive, were unsafe. One went on to fix her problems. The other i haven't dived with again.