I think there's another non-sports reason why Americans get excited about the World Cup, though. In fact, it's the same reason why I get excited about it so much: patriotism. Tens of thousands of fans from countries all around the world wave their flags, paint their faces, and cheer their countries on in ways that go far beyond the support they show teams in organized sports in their respective countries. It's very easy (and exciting, in my opinion) to get swept up in patriotic fever for a few weeks; it happens every time the Olympics come around, too.
I'm a fairly patriotic person. I love seeing the American flag wave in the wind. I sing along softly to the Star-Spangled Banner every time I attend any event where it's sung. I love celebrating July 4th every year, and try to keep in mind the true relevance behind it; it's far more important than simply a mid-summer holiday to have a cookout and watch fireworks.
If I had to choose a single sporting event that I'd want to experience live and in person just once for the rest of my life, without question it'd be either the World Cup or the Olympics. It's not so much about the events themselves (though seeing the Olympics in person for the events themselves would be pretty freaking awesome), but the opportunity to wave the American flag and show pride in my country. It might a little cheesy, but the athletes competing in these events clearly are excited about representing their countries as well, and that's an awesome thing.