It has one of the best scripts ever written for a musical, combining political issues, history and fast-paced comedy. The veteran director and co-writer of the show, George Abbott, was famed for inventing all sorts of ways to keep a play moving faster; he was obsessed with never wasting time for a second, and came up with ideas like covering scene changes with a short scene in front of the curtain. (So if there's a big scene change, instead of making the audience wait for the next scene, he'll have two characters walk across the stage discussing something or singing a reprise until the next set is ready.) It's amazingly fast and funny for a what is basically a serious story about politics, and it also incorporates all the musical-comedy conventions later "serious" musicals would do without: a secondary romantic couple, dance specialty numbers (choreographed by Peter Gennaro, formerly Jerome Robbins' assistant). And the score has a unique sound that fits the story perfectly, a sort of hard-driving, punchy, tinny sound, played up by Irwin Kostal's orchestrations -- the overture begins with a police whistle. It may be the best "Serious Musical Comedy" of all time.
And, though I've posted it before, one of the best comedy songs ever (and only one of several great comedy songs in the show; how many musicals with serious subjects would have so many comedy songs and so few introspective songs?):