Certainly we can take lessons from The Grapes of Wrath just as we can take lessons from almost all literature. They may not be the lessons specifically intended by the author, but that is also part of the beauty of reading it: we take away from it what we feel is important as individuals.
From my perspective, one of the lessons of this novel is that in today's world, even with this stubborn recession, we don't know how good we have it. The Great Depression redefined poverty. Americans actually starved to death. Today's situation is difficult, to be sure, but it is still nowhere near the level of misery and dislocation as the setting and time period of Steinbeck's tale.
I would also take away the lessons of compassion - that of giving to your fellow man when they are in need, even if you have little to give. There are numerous examples in the book of this taking place.
As a third lesson, I think the novel highlighted how poorly we treat our fellow Americans at times, discriminating against the poor, but also against people from certain regions, from the South, or in this case, against the "Okies" like the Joad family and their rude welcome into California.