Anvilicious: As stated above (Steinbeck was a socialist). Though very much Some Anvils Need to Be Dropped, particularly as economic conditions in the 21th century continue to deteriorate, with the working class getting the worse of it.
Everyone Is Jesus in Purgatory: Steinbeck stated that there are five levels to the story. Presumably, one is the tale of the Joads themselves, one is the Depression, one is man's inhumanity to man in general, one is why there are so many biblical references, and one . . . Who knows?
Also there's the uncanny resemblance between Jim Casy and Jesus Christ.
Harsher in Hindsight: The book has many themes that are still relevant today. Especially for the U.S. which suffered a heavy recession in 2008 and is suffering from water shortage and other agricultural problems that are making lives difficult for many. It's arguably because of this that the book is still banned in many schools and other places to this day.
Although in the near-to-medium term, World War II led to massive expansion in manufacturing and a decades-long building boom in Southern California. Not that anyone knew that in 1937-40 when Steinbeck was writing and the movie being made.