I know everyone is talking about this so my two cents are as follows - The King rocks!
As an American the whole thought of having a King is just so foreign and quaint and European. It is exotic. Of course, if I grew up with a King in charge (sort of) I may not like it so much. I'm also not even sure the Monarchy is going to last another fifty years in Spain - but this sure helps it along!
I think what Juan Carlos does do is show the people that all this bullshit they are usually arguing about - PSOE vs. PP, what happened in the past in that goddamn civil war, everybody pissed off over ideology, etc. is fine... just don't forget we (not me, them) are all Spanish (Yes, even the Catalans). This is where (I like to think) the Americans are different. We fight and argue a lot but in the end we know we are all Americans. We will close ranks - Democrats, Republicans, different colors, etc. - to an external foe. To his credit, Zapatero did do his best to defend Aznar.
Another thing.... The Spanish should understand that in Latin America they are seen as the imperialist interlopers past and present - just like the Americans are. There really isn't a lot of "brotherly love" toward the Spanish down there. Many South Americans (especially native peoples) hold a grudge.
I also love how Fidel Castro immediately backed Hugo Chavez even though Zapatero has done a lot to snuggle up to Fidel. Even Castro thinks that in the end, Spain is the imperialist enemy, just like another democratic country on his mind.