One of the ways Atlanta may differ from other major US cities is that it is located within the Bible Belt. Therefore Sunday is very much considered a "church day" whether or not church is attended. It affects how personal and business plans are made on a Sunday. With Atlanta's rich history this fact alone is interwoven into many layers of Atlanta's society and business culture.
Southern Hospitality is alive and well in Georgia. This means that people are friendly when they meet you and want you to feel welcome. There are smiles and greetings galore and even though Atlanta is technically a city, we have the benefits of country-living infused into our culture. For those who are better acquainted with a more "what you see is what you get" environment, this may appear strange, but in essence we feel being pleasant and generally adhering to political correctness in society and business is beneficial for building long-term relationships with people from different backgrounds.
In Atlanta, rush "hour" runs for a few hours in the morning (6 AM - 10AM) and the evening( 4PM - 7PM). Atlanta's neighborhoods are spread out from each other and consequently, we spend a lot of time driving. It is therefore important to plan around the rush hour. See the Travel 2 Atlanta page for more travel-related information.
The US in general is a much more litigious society than Britain and as such, one should be familiar with the law as it relates to your areas of interest while in Atlanta. It is also worth mentioning that Georgian law allows citizens to own licensed guns. It is therefore not unusual for people to keep guns in their cars.
For additional information, see Trip Advisor's article on Atlanta Culture.