Unlike the original book, the Once-ler is shown fully in the story as a human. Executive producer Christopher Meledandri said of the change, "The minute you make the Once-ler a monster, you allow the audience to interpret that the problem is caused by somebody who is different from me, and it ceases to be a story that is about all of us. Then it's a story about, 'Oh I see, the person who led us into the predicament is not a person. It's somebody very, very different.' And so it takes you off the hook."
When the Once-ler is walking to his office once he became rich, what appears to be a blow-up magazine cover is hanging on the wall. It is caption "Too big to fail", which is an colloquial term in economics to define an institution whose failure would bring catastrophic consequences to the economy. After the 2009 banking collapse, modern economists have said that institutions too big to fail are simply too big to exist.
The original name of the town in the movie is Greenville (as can be seen in the scene where the Once-ler is selling his thneeds in the town center, "Greenville Circle") before the commercial success of the product changed the name to Thneedville.
The trivia item below may give away important plot points.
The soundtrack reflects the natural vs. man-made nature of the scene. For example, the opening song "Thneedville" describes the thoroughly artificial town with completely electronic music. Meanwhile, the finale "Let It Grow" and the planting of an all-natural tree is made with almost all acoustic instruments. "Let It Grow" even includes a capella choral music, i.e. with no man-made instruments at all.