Without the US involvement in the war, it would have probably dragged on for years with the USSR and Germany bleeding each other to death. Without the US intervention, the Nazis would have been able to better support its forces in Russia and the major battles in Leningrad and Stalingrad would have gone on for years.
Hitler did send U Boats to the East Coast and attacked shipping lanes in the Gulf of Mexico. The Nazis did not have enough naval power to be able to cross the Atlantic and launch a full scale invasion of the US. But if they were successful in their conquest of Western Europe, an attack on the US would have been certain.
As for the economics, WWII helped the US emerge from the Great Depression and set the stage for one of the most prosperous decades in US history. The GI bill did more to establish the middle class in the US by sending millions to college who would have never been able to go but for the bill. It also set the stage for the US to be a world leader in industrial production, something that would later get pissed away by neoliberal globalization.