Neither Leif Eriksen nor Columbus ever got any near the territories that later became the USA. I imahine so far as the rest of the Americas were concerend it wouldn't really have made a great deal of difference if those territories had all been under water, up until the 19th century.

Take the USA out of the picture and it would meant most of the people who emigrated, including political dissidents, would have stayed home, and Europe would have had a very different history. Imagining a world without the USA but with everything else the same - Stalin, and Margaret Thatcher is just lazy alternative history.