Good traits of "nice guys":
they are considerate of others' feelings
they won't piss off your parents
they feel obligated to bring home a paycheck
they'll walk your chihuahua for you without complaint

Bad traits of "nice guys":
they care too much about what other people think
one day, they might go postal because they've been bottling up their feelings
they lie all the time to keep the peace
they can be boring cause they work too hard
they can be sellouts cause they want to be good citizens

It goes both ways on nice guys. It's better to be a guy with integrity who will take the truth as the authority and not the authority as the truth.

But that being said, being an asshole won't get you very far with women of quality.