Look at the world as it is today. How much of that view changes without the United States? Screw the things that can be seen as either good or bad, depending on persepective. Let's just go through the broad strokes.
Without a strong, unified United States, John F. Kennedy's reaction to Soviet missle placements in Cuba never occur. Marxist Communism isn't held back from the Western world.
Continuing with the "JFK" theme, there was never a mandate that the U.S. Space Program reach the moon within a decade. Russia wins the space race, and continues to develop technologies that revolutionize multiple fields. But, it's the Soviet Union, so thos things never see the light of day, because those advancements would be seen as detrimental to the State.
Going back a bit farther, in 1942, the United Kingdom was, for all intents and purposes, the only nation still in a position to fight the Axis advance in Western Europe. Without the United States, the UK would eventually run out of supplies, and be forced to retreat to Britainia. That is, if the Axis didn't just send the air fleet to wipe them out, entirely.
On the Pacific theatre, without the United States, the Chinese military never receives any support against the Japanese invasion of the mainland. Japan, with a vastly superior Air Force, wins the war on that front, adding almost a billion (at the time) people to its population. That's right. Without the United States, the Axis wins World War II.
But, then again, without the United States, World War II may never have happened. Think about that, for a second. The primary catalyst behind World War II was Germany. Germany was led to action by Adolf Hitler, who was using the Jewish people as a scapegoat for what was enacted by the Treaty of Versailles. But, in 1917, England was in the same position in which they found themselves in 1942. They, for all intents and purposes, were alone, against a vastly superior force, and fighting outside of their supply lines. So, without the United States, Germany wins World War I, meaning that World War II may have never occured.
Is that enough? Or, shall I go into the advances in medicine or technology that never come to pass without American ingenuity? I know that some people around the world don't like to admit this.... Hell, some people in the United States don't like to admit this.... But, the world IS a better place because of the United States. I'm not saying that we're "The best country on Earth." But, I am saying that history has proven that far more people have seen their lives measurably improved by the United States than have seen their lives harmed by the United States. It's really not even close.