Genre
Neo-Noir, Whodunit, Buddy Movie
Back in the mid-1990's, people were in love with cool, nihilistic movies with a high body count and reckless anti-heroes at the center—like Reservoir Dogs, Pulp Fiction, and (that's right) The Usual Suspects.
These neo-noirs took the basic plot structures of older film noirs—like, group of criminals get together to pull a heist—and added more violence, more cussing, and tricks and quirks. (The ending of The Usual Suspects is a good example of one of these tricks.)
But The Usual Suspects is also a classic "whodunit"—a mystery in which we try to figure out—yup—who the real criminal is. Was it really a mysterious, arch-criminal like Söze? Or was it someone more mundane, like Keaton? The ending to a "whodunit" would be pointless if it weren't shocking—and Usual Suspects definitely delivers the shocking goods.
Finally, it's a buddy movie—because these guys are buddies. They commit crimes together, but they also bro-out. We see them playing pool, getting to know each other in a jail cell, burying Fenster in the sand, and braiding each other's hair (not really, but Keaton would look stunning). We hang out with these guys and get to know them—which makes it kind of sad when they (mostly) die.