Tossing trash outside a proper receptacle, yelling at their kids/spouse in public, lack of emotional control at others expense, talking down, bragging, bullying.
I know it's far reaching and vague, but it's what I key in on.
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Actually, it's not that bad. You can tell who is just having a bad day vs. those who simply live like that all the time. Also, there is the effect that those who don't stand out don't get noticed, so I imagine that the people who qualify for douchebag status (as per me) is actually a very small percentage.