How is Our Town a modern American play?

Thornton Wilder's Our Town is not only a masterpiece of American drama but one of the earliest and finest works of modernism in theatrical plays. The play, which is set entirely in Grover's Corners, New Hampshire, focuses heavily on the passage of time and the infinite transience of life.
Our Town finds its roots in modernism through many characteristics of the play itself, most predominately its minimalist stage settings and theatrical style, its focus on the universality of ordinary American life, and the narrator's frequent engagement with the audience.
At the beginning of the play, the narrator—also known as the Stage Manager—speaks directly to the audience. While a technique similar to this had been used for centuries before (particularly in Richard III and several other Shakespeare works, in which a character shares motivations, thoughts, or sly comments with the audience) it is used here in a unique way. The Stage Manager describes the world of Grover's Corners, sets the particular date, introduces several characters, and sets any necessary backstory through exposition. This is a modernist technique that strays from the traditional narrative forms of theater, drawing instead on traditions found in written novels with omniscient narrators.
The bare and minimalist stage settings require active participation from the audience in the form of imagination. Where other plays may utilize extravagant costumes and lush theatrical sets, Our Town transcends the particularity that a crafted set might offer.
Through its many inventive and bold approaches to theater, Our Town was an early example of the countless opportunities that creative freedom would offer in American theater. From the grounds laid down by Thornton Wilder and Our Town, many other modernist American plays, such as Death of a Salesman and The Glass Menagerie were able to sprout.