Very good tips, I will give some tips too ( I have been back in fourth in this country for 5 years now, but living here for the last 2 years).
Don't walk around with your camera taking pictures, it doesn't matter if you are in zona colonial. I see tourists doing this all the time, I remember saying to one to keep his camera on his bag, he ignored me, few hours after when I saw him, he told me he got robbed. I was not surprised.
Always call the taxi, don't pick it up from the streets, you will pay more this way and is less safe.
Don't believe girls that tell you that they are in love with you. Dominican girls have really good game and you as a foreigner to find a decent one that likes you for what you are is extremely hard. Understand that many of them are trying to leave the country.
If you look like a local, the country is not that dangerous but if you look too much of a foreigner, be aware of your surroudings and watch where you walk around.
Dont misinterpret niceness for goodness, a lot of people in this country are nice but they are not good. Don't get me wrong, you can meet some good dominicans but unfortunately not many of them, this is a very hustling society, they believe that in order for them to get ahead in life, somebody needs to be stomped on.