I'm a Democrat, as I believe I've mentioned somewhat incessantly. And one of the big inspirations for my family was John F. Kennedy. It's one of my Dad's big regrets that he voted for Nixon in 1960 and not Kennedy, but I think that's pretty much the last time my father voted for the Republican candidate for President.
I can't speak for my Dad, but I know what attracted me after the fact (given I was born after his assassination) to JFK was his clarion call to serving our great country. The famous quote was, of course, "ask not what your country can do for you -- ask what you can do for your country!"
JFK made this call to service in several contexts -- in the fight against Communism; in the fight against racism on behalf of civil rights; in his call to get a man on the moon by the end of the 1960s; and finally in asking young, idealistic Americans to serve all humanity in eradicating poverty via the newly-formed Peace Corps.
It is the fact that JFK called us to serve more than just ourselves, that made him so beloved not only in this country but across the world. Time and again when discussing my country with folks from other parts of the world, whether it's my mother's native Germany, or Russia, or with many colleagues I worked with from India, JFK was brought up favorably, even reverently, in stark contrast to the current crop of politicians in America.
Obama clearly has the JFK style down: the ringing rhetoric, the air of cool, the adoring crowds. The critical thing he lacks is the substance that made JFK more than just a callow, pampered son of a wealthy family.
If I were to boil down Obama's message to its essentials, it would be, "what can America do for you?" Withdrawal from Iraq, sure! Healthcare for all, you got it! Green energy without having to continue any nasty drilling for oil, piece of cake. And don't worry about sacrifice, my fellow Americans, because only the rich will have to do that, and as you're not one of those, you won't have to worry about it. So sayeth Obama.
McCain, by contrast, is all over the "ask what you can do for your country" side of JFK's formulation. He is certain of a bright future for America, but he knows it's going to take more than magical, fluffy bunny thinking to get us there. It's going to require that we all transcend ourselves, that we will have to pull together and work hard, and that some of us may even fall in struggle against our foes, but that this is all well worth it when we serve America's common good. Indeed, McCain explicitly states that "I do not claim to save my country, for I know my country saved me" when he was a POW.
There is a corollary to this, and that is that McCain is far more willing to tick people off than Obama is. Just McCain's own fraught relations with his own Republican Party shows this. Obama tries to portray a world in which everyone is pretty much in agreement, and no one needs to get upset or hurt along the way to the brighter future.
McCain will not mince words for the most part, and if his campaign finance reform efforts ticks off some conservatives, oh well. If his blunt formulation of marriage being between a man and a woman upsets some, well, that's the way it is. Or for that matter if Russia's tender feelings are upset by being labeled the aggressor in Georgia, well, that's just too bad. And I write this as a pro-choice, pro-marriage equality dude, but I respect McCain's character even where I disagree with him.
This may be an inappropriate comparison, but I'm going to make it anyway: think on the friends who've been with you through thick and thin, who you know no matter what you can count on. Are they ones that always tell you're doing okay even when it's clear you're on the wrong track? Or are they ones who make it clear that, while they'll always be there for you, they will call you on your B.S. when necessary?
Obama is that bland sort of friend who will never call you on your stuff for fear of annoying you, and McCain is the warrior on your side who may irritate you, but he will challenge and push you to be better than you are.
That's why McCain has what it takes to be a leader, and Obama manifestly does not.