Thanks again to Steph for provoking me to a post that's challenging to write.
So, where did St. Francis get all his ideas? From what insights did the Franciscan movement emerge? A lot of ink has been spilled on this issue, but you might start at Cajetan Esser's Origins of the Franciscan Order, if you can find it.
For a quick and dirty answer, I suggest 4 categories: apostolic life, class, violence, and money.
1. The apostolic life. The 12th century was a time of great energy around religious life and the experimentation with new forms. In contrast to the stable life of the monastery built around the communal, primitive church imagination of Acts 2:42-47, people began to long for the life of the apostles themselves. This meant itinerant preaching, homelessness, and the economic insecurity that goes with them. This insight produced a lot of religious movements, including the Franciscans. The followers of Peter Waldo are a good example of an expression that came before Francis.
2. Class. A shift in social class structures was going on at the time of Francis, especially in Italy. The re-invention of the money economy was producing a merchant class, shifting power away from the church and the ruling classes to the towns. Francis himself was part of this nouveau riche merchant class. As a child, Francis presumably witnessed the sacking of the imperial fortress of Assisi by this new class of urban elites.
Even more, the young Francis dreamed of his own upward mobility. He would become a knight. This imagination never really leaves Francis, though it shifts into his chivalrous affiar with Lady Poverty.
From his experience of these chaotic changes, it is easy to see how Francis could have imagined his brothers as fratres menores, identifying them with one of the older social categories, those who were "lesser" in society. Francis saw the emergence of new forms of power and appropriation. With these came new forms of economic and class injustice. Thus he wanted his brothers to be "lesser" brothers who lived sine proprio, without property.
3. Violence. Francis was born into a very violent world. I've already mentioned the destruction of the imperial fortress by the people of Assisi in 1198. Francis himself was taken prisoner in one of Assisi's savage wars with neighboring Perugia. His biographers tell us that his conversion began to take place during his convalescence after spending a year in the Perugian dungeons.
It was perhaps from his own experience of violence that Francis began to identify with the ultimate victim of violence, Christ crucified. In any case, he soon rejects all the power and property that leads people to need weapons and protection.
4. Money. The new economy was changing everything in Francis' time. It changed relationships, upset the old order of things, and created new forms of poverty and wealth. Francis became almost pathologically opposed to money, and was always forbidding his brothers to receive it or even to touch it (with the exception of caring for sick brothers in the Earlier Rule.) For Francis, money was a concentrated form of impersonal power, and thus a threat to the dependence on God and others that was the life of the lesser brother.
O.k., that's a lot. But notice! All of these motivations are even more true today! That's why the Franciscan movement continues to have something to say to the world.