Consider anything drugstore-related a must-take to the Dominican Republic , especially if you are staying in a resort, as prices are highest there. Don’t leave home without your prescription medications.
Lightweight fabric is a good choice for this warm climate, but take a warm layer for chilly nights near the ocean. Overall, the mountains are much cooler; pack warm and expect rain in the winter.
Diversity of landscape and activities signals diversity of footwear: sandals for beaches, boots for hiking, dress shoes for nighttime. Consider water shoes for wallowing in the surf to avoid cutting your feet on coral beds or stepping on sea critters.
Dominicans dress up to socialize. Jeans and T-shirts are sometimes too casual for evening, and many discos and restaurants have strict dress codes (especially in the cities). Daytime sightseeing calls for light clothing but never beachwear. Many historical sites and churches have strict rules about what is appropriate attire. For instance, going into the cathedral in the Ciudad Colonial  wearing shorts, flip-flops, and a tank top is considered disrespectful.