USE? Great acronym...
True, the blood runs a lot deeper in European soil than American soil (unless you can't that whole indian/civil war thing... sheesh, we really don't get along that well).
I think the biggest barrier to European unification is language. Granted, most of them are bilingual at least, but the primary language is different, and that can be a major seperator.
Also, there are much stronger Protestant/Catholic strands in Europe than there were in America. We were designed to be a secular state, where many religious Europeans, especially the Christian parties, would see that as a huge threat.
Finally, there begs the question "What is Europe"? What nations would partake? The economies of Western Europe and Eastern Europe are hardly compatible (this will be a mess for the EU).
But... I think European unification will happen eventually. Not because it was forced on anyone, but because after so many years of cultural exchanges and peace the necessity to seperate slowly disappears.
It'll be a while though... considering how far Europe has come since WW2, I might see it near the end of my lifetime.