Show Boat is generally considered to be the first true American "musical play" — a dramatic form with popular music, separate from operettas, light musical comedies of the 1890s and early 20th century (e.g., Florodora), and the "Follies"-type musical revues that preceded it. In many ways, it took the plot-and-character-centered "Princess Musicals" that Kern had developed with Bolton and Wodehouse in the previous decade and broadened their scope.
Of Kern himself, I've heard it said that he had one foot in the old world - that of the 19th century European operetta style - and one in the new - that of a purely American style of songwriting. In that sense, Kern bridged the gap between Gilbert and Sullivan and Richard Rodgers, between The Pirates of Penzance and Oklahoma!. His familiarity with the old Victorian high-style of musical theatre and songwriting probably had a lot to do with the pure prettiness of his tunes. There is a sentimentality to his melodies that harken back to the old days, a sentimentality you would never find in a Rodgers tune, or a Gershwin. The titans who followed Kern on Broadway in the years after Show Boat - Berlin, Rodgers, Gershwin, Cole Porter, etc. - scrubbed their music clean of the old-world emotions and crafted a style based, both musically and lyrically, on wit, self-control, a relaxed charm, a serene nonchalance. There was a emotional distance to their music, and they were never mawkish. Only occasionally would they attempt to go deeper emotionally, Berlin for instance in a song such as "What'll I Do?", or Cole Porter (who, along with Richard Rodgers and Larry Hart, epitomized the newer style) in songs such as "So In Love" or "After You, Who?" But mostly they stayed away, appropriately, as they crafted those utterly new and interrelated art forms called American Musical Theatre and American Popular Song. Along with jazz, rock and roll, and the movies, they are this country's enduring contributions to 20th century entertainment.
Kern was a contributor and an innovator of this form. Today, we remain in debt to him for Show Boat, and for some of the most memorable melodies we know; "They Didn't Believe Me", "Smoke Gets In Your Eyes", "All The Things You Are", "Long Ago and Far Away", "The Song Is You", "The Last Time I Saw Paris", "Ol' Man River", "Can't Help Lovin' Dat Man", "A Fine Romance", and, probably the most recognizable song in his canon, and one of the most romantic songs ever written, "The Way You Look Tonight".