Just to clarify, our territories do not actually have electoral votes in Presidential elections, nor do they have voting members of Congress. I would say that the US has been an empire for well over a century, Americans simply don't like to think of themselves as belonging to one. One thing that is unique about our empire, though, is its unprecedented economic influence. It represents 1/4 of all the business on Earth. Three other nations use the US dollar as their official currency, and in Cuba, a country that doesn't like us that much, it is still the dominant unofficial currency there. The US is able to, through imperial economic power, secure 25% of all world oil production and use, even though it is only 5% of the population.
The US offers electoral votes to its "colonial" holdings, Puerto Rico, Guam, etc. This might be unique.
The most well-known non-state US holdings are part of a military strategy and not part of an economic policy. That may be unique.
The United States is unique because it began as a colony of an empire, but developed into a powerful nation. The United States is not really an empire in itself, but it does have two protectorates: Guam and Puerto Rico, and states that were once sovereign, like Alaska and Hawaii.
The first three posts' argument that we're not an empire because we don't have heriditary rule is kind of weak in my opinion. The British didn't truly have heriditary rule by the time of Queen Victoria and no one claims they weren't an empire.
I still don't think we're an empire, though. We don't have true control over other countries. We may be hegemonic, but we do not actually have control over the other countries.
Anyway, what makes us at least somewhat different than empires that have gone before is that our "empire" exists in a more enlightened time when empire is not acceptable. Therefore, we try to make our empire be a help to those we influence. We do not always succeed, and we do at times do things for our own power. But we also truly do have a much stronger impulse towards things like "nation building" than any other empire has ever had.
There are few nations in the world that are comprised of such widely divergent people, and where each and every citizen--no matter their heritage--remains equal in the eyes of the law. The United States is also unique in the manner in which the government has decided to make the nation the World's Policeman, ready to forcibly quell any disturbance that serves in our best interests.
There are several things that are unique about America. First, as you know America is a great power in the world. However, from a geographical point of view, there are no powers near it that feel threatened. The other powers in the world are near many rivals. For example, if China becomes too powerful, it sends a red flag to Japan, India and Russia. If Iran becomes too powerful, then it send a red flag to Israel and Saudi Arabia. In relative isolation of America make it unique.
I would agree with the above post - I don't think of the United States as being an empire, although some might argue that it may be moving toward an oligarchy with a few interconnected business interests in control.
As a byproduct of our geographical location in the world, I think one unique factor in United States history has been our role as an outsider going to areas when we become militarily or otherwise involved in another country's affairs. I think our national attitude toward foreign relations has been significantly affected by the distance between our country and most of the rest of the world.
I would agree with post 2. While the US is a vast region compared with other countries, I would not consider it an empire. I think of empires as falling under the rule of one person. The Romans had Cesar. The Ottoman's had sultans. I do not think the democratic government of the US can be seen as the head of an Empire. Yes, there are many states that comprise the US. I can understand why this might seem similar to the many countries typically making up an Empire, but it is not the same thing. The US does divide the legal system between state and federal levels as an Empire might allow each country a certain amount of autonomy. Again, the state system in the US is not the same thing as separate countries and counties making up an Empire.
I would not necessarily consider the United States an empire. An empire is denoted as either a monarchy or an oligarchy. Given that the US is not a monarchy (ruled by one who the title is passed down to) or an oligarchy (a government controlled by a small group, typically a family), I would not consider it an empire. Some may disagree with me based solely upon my understanding of the three terms.
One might note that as stated in post #7, the Untied States began as a group of "widely divergent people." If we believe Plutarch in his biographies of early Romans, the same is true of Rome.
Post # 9 says that the United States began as a colony of another country. One might say it was colonies of Britain, France, Spain, Russia, Netherlands, and for good measure one might also add the Sioux and some other North American nations, which controlled large parts of what has become the United States.
One might note that Carthage began as a colony of Tyre.
If the United States is an empire, it is a strange one indeed. One can see that by looking at a list of American conquests. Algeria (the shores of Tripoli), Cuba, Germany, Grenada, Haiti, Japan, Mexico (the halls of Montazuma), Panama, Quebec, the Philippine Islands, and Texas have all been controlled by force of American arms. Texas is the only one that is still part of the United States.
A British Historian, Niall Ferguson, likes to call the United States is an empire. I have not read his books, but I have seen him in CSpan videos. He's interesting even if his notion of empire is a bit nutty.