While the show is known for creating an awareness of European modern art in the United States—it was originally planned to just feature American artists, and American art (even with the European inclusion) still outnumbered European art 2 to 1 in the New York exhibition.
The Armory Show was not the show’s name. Maybe an obvious point but it was called “The International Exhibition of Modern Art.” The "Armory" name was unofficially applied and was due to the show's initial housing in New York at the 69th Regiment Armory.
The American art world was more progressive in 1913 than is usually acknowledged. Two major reasons: the 1908 exhibition of “The Eight” artists who became known as the Ashcan (or "Ash Can") School and Alfred Stieglitz’s Photo-Secession exhibitions.
The artist with the most works in the show? Odilon Redon.
The Armory Show is most often associated with New York. However, it travelled to Chicago (to The Art Institute of Chicago) and Boston, and its attendance in Chicago more than doubled that of New York. (Even though in Chicago there were roughly half as many works on view as there were in New York.)
Almost half of the people who bought works at The Armory Show were women and important (women-led) collections of modern art arguably began with The Armory Show, including that of Lille P. Bliss, whose collection was of primary importance to the 1929 founding of The Museum of Modern Art, New York.
The artists most associated with The Armory Show—Marcel Duchamp, Pablo Picasso, Francis Picabia, and Georges Braque—were actually stuck in a back gallery—the so-called "Chamber of Horrors"—that was in a far corner of the New York exhibition hall.
Paintings in the show were occasionally hung salon-style or "skied," i.e. one above another. This may be obvious to say but it’s interesting to note how rarely this is done now, especially for larger works. The reasons behind this change include the growth of exhibition spaces and the increasing value of works of art, as well as the influence of museums, such as MoMA, who created sparser exhibitions. For more on MoMA’s influential exhibition practices, see Mary Anne Staniszewski’s The Power of Display.
A pamphlet published while the show was in Chicago by the show’s organizers presented reviews both for and against the exhibition. For more primary documents related to the show, see the Archives of American Art website devoted to The Armory Show as well as Bruce Altshuler's important book, Salon to Biennial, which presents significant exhibitions in modern art (a major untold story of art history) between 1863-1959.
One of the most often-ignored reasons for the rise of abstraction was the rise of photography. Alfred Stieglitz notably expressed his belief in photography's influence on abstraction at the time of the Armory Show. He described the painters and sculptors included as “a score or more of painters and sculptors who decline to go on doing what the camera does better.” He added that, after seeing the show, “you’ll go back to your habitual worship of eternal repetitions of mere externals of people and things that cram all the museums and galleries—but you won’t feel happy. The mere outside of things won’t satisfy you as it used to.”