Keith R - You're splitting hairs. The short form is not based on nouns or adjectives, its based on communication and what people will understand. Calling it the States will allow better communication with someone than calling it the United. Calling it the Dominican will allow better communication than calling it the Republic - there are a lot of republics!
Shortening the official names of nations is commonplace, and I offer you yet another possible explanation for its widespread use: I feel it does not imply a lack of intelligence, nor an intent to insult. Perhaps it is not only that it is easier to say/type in alternate forms (US, States, America, United Sates, United States of America) but people also tend to shorten names in order to imply familiarity. I grew up in Prince Albert, but we are from P.A. as far as locals go, and anyone else that knows about the town.
People that have been to the island or have fallen in love with it without even seeing it will perhaps call it the Dominican not only because it's easier, but because they are trying to imply familiarity with it. Maybe it's to impress friends, maybe because it makes them feel part of it, maybe it's because it easier to say - or a combination thereof.
As for Nelly, we're 100% aligned on this one; The proper name is the Dominican Republic, but to get so worked up about people calling it the Dominican - I don't know. Sounds like a symptom of people with very little to complain about in life.
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