Adventure, Historical Fiction, Folklore, Coming-of-Age
First and foremost, Seven Samurai is about excitement. Thundering warlords come galloping down on top of our heroes as they engage in a life-or-death struggle to save the innocents from oppression. Along the way, they plan ambushes, burn down the bad guys' fort and even take a couple of hapless guards by surprise. We call that "adventure time" around here, and considering the number of subsequent action films it influenced, we don't think anyone's gonna disagree with the assessment.
At the same time, Kurosawa is very clear about when this takes place, and the historical time period helps explain why the folks onscreen are doing what they're doing. Things were bad in the Sengoku period, with a weak central authority providing as much protection to the people as a wet paper towel, and a lot of formerly noble samurai wandering around without a lord to call "master." That lawlessness had a certain romance to it. Kurosawa wanted to see that there, as well as reminding the Japanese that they once went through another bad period in their history, and that they need to deal with this new one the way their ancestors did the old.
And as realistic as it is, it also takes on the vein of folklore: the kind of tall tale you'd tell around the campfire to impress your buddies at summer camp. There's a certain elegance to the way the drama unfolds. The magic number seven is called up, and with the samurai as wondering knights, it's not much different than King Arthur and the Round Table. Granted, Kurosawa hits it all with a dose of his nation's culture (and sticks firmly to realism throughout), but the underlying vibe definitely has that "once upon a time" sheen.
Finally, we'd be remiss if we didn't point out Katsushiro's big arc, which follows all the patterns of a coming-of-age story. He finds a master to tech him the ropes, falls in love with a girl that he knows he can't be with, and otherwise learns the sad and sobering ways of adulthood. If he were the only guy in the movie, it would definitely be a coming-of-age story; even with him relegated to sidekick status, you can't let the film go without commenting on it.