"The Star-Spangled Banner" is more or less officially in three-quarter time. Printed sources of "The Anacreontic Song," the drinking song that supplied the tune, are in triple meter, as are other, pre-"Banner" songs that use the tune (such as "Adams and Liberty"). Here's the traditional version:
The Brooklyn kids sang something more like this:
Basically, each downbeat is doubled in length. What it does is make the anthem sound like a gospel song.
Spreading a dactylic rhythm over four beats like this shifts the rhythmic activity to the second half of the bar, which is common in gospel music. Here's the opening Richard Smallwood's contemporary gospel standard "Total Praise," which has the same three-stretched-into-four (or even eight) pattern:
It's prevalent enough that it's an easy way to give a piece a gospel feel, as in Billy Taylor's "I Wish I Knew How It Would Feel to Be Free," made famous by Nina Simone:
Long-short-short and related patterns are pretty venerable, so how did the pattern become associated with gospel? One might think it's through comparison with white gospel, which has always kept a healthy repertoire of waltz-time numbers—shifting to 4/4 doubles the backbeat quotient and makes for a more jazz-like rhythm. But I think it has more to do with the call-and-response pattern typical of African-American churches: the long downbeat opens up space in the bar for the choir/congregation to interject. Here's a good example from John W. Work's American Negro Songs:
I'd be willing to bet that the common-time, gospel-tinged variant of "The Star-Spangled Banner" has its origin in the African-American civil rights movement of the 1950s and 60s, but the first such public performance I know of was José Feliciano's soulful prelude to the fifth game of the 1968 World Series. (The famous Jimi Hendrix Woodstock anthem was in a solid slow three.) At the time, Feliciano's version was widely regarded as disrespectful; within two decades, Marvin Gaye's 1983 NBA All-Star Game rendition was an instant classic. Nowadays, at one of the most solemnity-fraught public observances on the calendar, a 4/4 "Star-Spangled Banner" doesn't even raise an eyebrow.
There's a lot of reasons why. Part of it is the prevalence of pop styles in the current American musical landscape—those hippie teenagers from '68 are now running the place, after all. You could argue that genres, by now, have criss-crossed so many boundaries that the mere use or implication of a style doesn't say much of anything vis-à-vis the song at hand (Gaye was singing soul, which was secularized gospel, which in turn was churchified jazz, which evolved from ragtime and blues, which grew out of spirituals, etc., etc.). I think there's two particular things going on here, though.
The first is that our expectations for patriotic feelings have changed. In a way, the National Anthem has shifted in performance to a sort of national hymn. (Francis Scott Key's original poem does reference the Deity, but not in the only verse that anyone knows.) Of course, there already is a national hymn: "God of Our Fathers," composed in 1888, and as stolid a piece of white Anglo-Saxon imperial Gilded-Age triumphalism as you'd expect, just the thing for reinforcing traditional American values of rectitutde and propriety. But such dignified piety paled in comparison with the heady energies of the African-American church, an experience that was firmly installed in the mainstream by the civil rights movement. Once we saw the view from that mountaintop, it became the ideal inspirational touchstone. We didn't just want the comfort of tradition and heritage. We wanted to be transported.
A fundamentalist straw man might argue that we're looking for a replacement for those religious facets of governmental ceremony that have allegedly been leeched out of public discourse by civil libertarians. (I don't think so.) A cynical straw man might say that we want the spiritual uplift of religion without the corresponding responsibility, so we've shifted those tropes to a non-religious context. (Maybe.) I would say that there's a more fundamental change at work here, a change in the way we perceive what "country" means. Stephen Decatur's infamous 1816 toast—"Our country, right or wrong!"—comes across as less jingoistic if you consider that Decatur's idea of his country was his camaraderie with his fellow countrymen: we may disagree, he was saying, but our collective bond as citizens should never be dissolved. We don't think of it that way today; our "country" is an ideal, a set of beliefs about government and freedom, a goal to be reached through collective conscientious citizenship.
Why do we think this way? It gets us into all kinds of trouble. If we clearly viewed our country as simply an assemblage of people living on a certain parcel of land surrounded by a border, I doubt we would be stuck in Iraq, for one thing—we wouldn't have seen the need to go there in the first place. But since we also see the country as this dream of liberty, this beacon of democracy for the entire world, we're less likely to notice such cognitive dissonance until it's too late. What's more, as time goes on, we're more and more likely to concentrate on that idealized country. Decatur and his fellow Americans thought that the perfect society envisioned by the founding fathers was imminent, just a few constitutional and economic adjustments away. We know better, which makes that goal all the more seductive in its permanent elusiveness. The great joy and tragedy of the American experience is that perfection always seems so close that you can almost, but not quite, reach out and grasp it.
The bulk of the Negro spiritual repertoire is focused in some way on the transition from this world to the next. Flipping through Work's collection, there's "After 'While":
After 'while, after 'while"You May Bury Me in the East":
Some sweet day after 'while
I'm goin' up to see my Jesus,
O some sweet day after 'while.
You may bury me in the East,"Lead Me to the Rock":
You may bury me in the West,
But I'll hear that trumpet sound
In-a that morning.
As I go down the stream of time"Steal Away":
(Lead me to the Rock that is higher than I)
I leave this sinful world behind
(Lead me to the Rock that is higher than I.)
My Lord calls me,...and so on. Heaven, in other words, is within sight, just over Jordan, which makes the consummation even more devoutly wished. No wonder our paens to our just-out-of-reach ideal country seem to evolve towards millenial religious utterance.
He calls me by the thunder;
The trumpet sounds within-a my soul,
I ain't got long to stay here.
Perhaps that's reading too much into things, but I think that the transformation of "The Star-Spangled Banner" into a gospel song, at the very least, signals another wish: the wish for a country worth singing a hymn to, the wish that the nation will finally cross over into the promised land that each of us carries in our mind. The longer the trip takes, the more we try and bolster our spirits. We sing to encourage ourselves, and our country, that we and she will get there someday. In the words of another spiritual: we cheer the weary traveler.