Dominicans and the DR in general are loud, and it is essential to not get overly annoyed at the volume of both the music and the voices. Often what we North Americans would mistake for an argument is only animated conversation. What LTSteve says is true about clothing but I see more and more Dominican men dressed in shorts and running shoes/flip-flops, even in banks.
Even attempting to speak Spanish is appreciated and many Dominicans understand some English, especially if spoken clearly.
Eating or drinking in front of people without offering to share is considered rude.
I've notice the Dominicans in Santo Domingo are louder, than the ones in Santiago.
I also notice many dominicans will order a sampler plate with many different items on it and everyone will share.
Onething that would not fly in America is bringing your own drink/bottle to a place that sells drinks. I still can't belive they get away with that.