While travelers from many cultures don one of their nicer outfits for the trip, Americans opt for comfort. As Sarah Lanier puts it in her book, Foreign to Familiar, there is something in the American mindset that says being comfortable is of higher importance than looking appropriate.
There is a reason for this, Lanier goes on to explain. It turns out that cultures that are generally more informal -- including not only the US, but also Australia, the modern state of Israel and Canada, for example -- are the younger countries who have had less time to develop age-old traditions piled high on top of each other. The older, more traditional cultures are called "high-context" cultures, while the newer cultures are "low-context'. Even in poor countries, the people dress their very best when going to a meeting, out in public or to someone else's home for dinner.
Americans, Australians and other low-context cultures are quick to address each other by their first names. Even though I've now been back in the US for fifteen years after living in Europe, I'm still taken aback at times how even in somewhat formal situations, such as in TV or radio interviews, the first name is used immediately.
Remember, we're not talking necessarily about right or wrong here -- but what is important is that the traveler be oriented to the host culture he or she is visiting. Otherwise, innocent mistakes will often be interpreted as insults. This works in both directions: the Korean culture, for example, is one of the oldest on the planet, therefore extremely high-context. Koreans immigrating to the US should be prepared for the shock of informal American culture. Many Koreans prefer to be addressed by Mr., Miss or Mrs. and their surname, and are often offended when immediately addressed by their first name.
The quintessential low-context culture is southern California, which explains why many people in California, whether natives or recent arrivals, feel a sense of freedom to be creative, start new trends, or be different.