In his talk last night, Fr. John Langlois, OP, brought out a couple of aspects of the historical development of the Rosary that were new to me.
One is that the Rosary devotion is a combination of two spiritual streams popular at the time the Rosary developed: a general devotion to Mary (think of all the cathedrals dedicated to her that were built in the high Middle Ages, or the preaching of St. Bernard of Clairvaux); and meditation on the life of Christ (the medieval passion plays, the living creches of St. Francis).
So you might say that
Rosary = (Life of Christ) + (Love of Mary)
You certainly don't have the Rosary as it's been known for five hundred years without both, but I hadn't thought about the naturalness of the Rosary emerging from the combination of both. (I'm not sure a Roman Catholic can have both -- that is, meditate on the life of Christ while expressing love and honor for His mother -- without having the Rosary. At the very least, it would seem an unnatural omission.)
Another aspect is the significance of the Rosary picture books that began to be printed in the late 1400s. Rosary devotees today often say the reason many Catholics downplay the devotion is that they, the downplayers, are snobs who regard the Rosary as too common and lowbrow. However many anti-Rosary snobs there may be, it does seem to be true that the Rosary is common and lowbrow. It's in the form it is today in large part because you could print a book with three woodcuts, or commission a painting with fifteen scenes, and people could look at the pictures. If you wanted fifty different meditations, or a hundred and fifty, as a practical matter you'd have to do it with words, and at the time reading was (relatively) uncommon and highbrow.
And once picture books began to appear, standardization of the mysteries happened quickly. It was less than a hundred years between the first picture book (printed in 1483 by Conrad Dinckmut of Ulm) and Pope St. Pius V officially defining the fifteen mysteries (1569). The only change the Pope made to Dinckmut's selection was the final glorious mystery: Mary's coronation makes a more cheerful conclusion than Christ's return in glory, with the Final Judgment that entails. (People who aren't sure what to make of Mary's coronation, though, can regard it as representative of the more general reception of all the saints, body and soul, into heaven on the Last Day.)
A thorough on-line source of information on the history of the Rosary in the Catholic Church, both the prayer and the physical set of prayer beads, is "Journaling the Bead." It naturally mentions the tradition that St. Dominic received the Rosary from the Blessed Virgin in 1208, although as Fr. Langlois points out this is a tradition that cannot be traced back earlier than the preaching of Bl. Alain de la Roche (or Alanus de Rupe), beginning about 1460. I've seen some contemporary attempts to defend the tradition, but the complete silence on the matter of all surviving sources for the first 250 years after the vision reportedly occurred makes for a mighty tough burden on the defense.