Accidentally Correct Writing: In the film, the main villain is charging people for clean air mainly because of there overly polluted air and smog, In recent years, an 'Oxygen bar' in Delhi gives Indians a gulp of fresh air. Low air quality has given a business opportunity to entrepreneurs offering pollution-free oxygen at a price.
Billing Displacement: The film's opening credits start with the three characters from the book in order of importance (Danny DeVito as the Lorax, Ed Helms as Once-Ler, and Zac Efron as Ted) and the rest following. The trailer on the other hand plays up Zac Efron and Taylor Swift, Ed Helms, and gives Danny DeVito an "And Starring".
Cut Song: There are a few on the soundtrack, as well as some alternate versions of "Thneedville" and "Everybody Needs a Thneed".
Executive Meddling: This is generally agreed to be the reason that "Biggering" never got animated in the movie, likely because they thought that it would be too dark for the audience and cause the movie to sell less. A song criticizing humans for being greedy bastards was removed by a few greedy bastards... ironic.
The Other Marty: In the Latin American Spanish dub, Jesse Conde was cast as the Lorax, and he even recorded the dialogue in advance. However, Danny DeVito, the Lorax's English voice, was eventually cast to dub over Conde's lines before the release.
In the Latin American Spanish dub, Memo Aponte Mille was considered to voice Ted before actor and singer Alfonso Herrera was eventually selected.
There was originally a different song that served as Once-ler's villain song instead of How Bad Can I Be. It wouldn't have been out of place in a rock opera, and was a fair amount darker and less goofy than the final product, showing that greed and pride were the reasons Once-ler went as far as he did.
Ted's motivation was originally getting something called a "Whosit", rather than wanting to impress Audrey (who apparently didn't exist at that point).
According to a demo of "Thneedville", the big blue sky painting originally didn't exist; the citizens were exposed to the smog-covered sky and aware of it. Also, there was a hill named after the city's founder (presumably the Once-ler), which was the only place that always stayed the same, but was soon going to become a pizza chain.
O'Hare was originally named "Oxy" (as in "oxygen") and underwent several designs. Ted also had several versions, some of which weren't as close to his book version. He also was seen with many strange accessories (such as giant gloves and unicycle/self-walking chairs, probably the "Whosits" mentioned above).
Word of God: On the DVD commentary, it is mentioned that after "How Bad Can I Be?", 5 years have passed.